302 core topics
+ 100 prerequisite topics taught
as needed · approximately 124 hours of instruction
including spaced review
An adaptive diagnostic (up to
40 questions) places the student on the course's knowledge
graph - topics already known are credited, and instruction begins exactly
at the learning frontier. Every topic is taught with a worked-example
lesson and auto-graded practice; a topic is mastered at
75%+ and then maintained through spaced reviews on an
expanding schedule. Mixed checks follow every 6 lessons;
each unit ends with a 12-item quiz, and course-wide assessments appear at
25%, 50%, 75%, and 100% mastery. Prerequisite gaps below the course are detected and taught rather
than skipped, so completion certifies the whole tower, not just the top.
| Word Equations
[M] |
Undo the operations in reverse. |
| Sum-and-Difference Systems
[M] |
Add the two facts to double the larger. |
| Proportions
[M] |
Equal ratios scale by the same factor. |
| Percent Change (Both Directions)
[M] |
A p% rise multiplies by (1 + p/100). |
| Averages and Totals
[M] |
Average times count gives the required total. |
| Average Speed (Equal Times)
[M] |
Average speed is total distance over total time. |
| Choosing a Group
[M] |
Order doesn't matter, so divide out the rearrangements. |
| Ordered Arrangements
[M] |
Multiply the choices place by place. |
| The Pigeonhole Principle
[M] |
Plan for the worst case, then add one. |
| Consecutive Integers
[M] |
The middle number is the average. |
| GCD, LCM and Products
[M] |
GCD times LCM equals the product of the numbers. |
| Remainders of Powers
[M] |
Remainders of powers repeat in a cycle. |
| Fraction Arithmetic
[M] |
Common denominators to add; straight across to multiply. |
| Angles in a Triangle
[M] |
The three angles always add to 180°. |
| The Pythagorean Theorem
[M] |
Legs squared add up to the hypotenuse squared. |
| Area of a Triangle
[M] |
Half of base times height. |
| Areas of Composite Shapes
[M] |
Add or subtract simple rectangles. |
| Circle Area and Circumference
[M] |
Area is πr²; circumference is 2πr. |
| Volume of a Prism
[M] |
Base area times height. |
| Summing an Arithmetic Sequence
[M] |
Average the first and last, times the count. |
| Geometric Sequences
[M] |
Each term multiplies by a fixed ratio. |
| Probability with Two Dice
[M] |
Count favorable pairs out of 36. |
| Counting Grid Paths
[M] |
Choose which steps go up. |
| Two-Set Counting
[M] |
Add the sets, subtract the overlap, subtract from the whole. |
| Age Relationships
[M] |
Set the future ages equal to the condition. |
| Mixture Ratios
[M] |
Find the scale factor from the part you know. |
| Working Together
[M] |
Add the rates, not the times. |
| Powers
[M] |
A power multiplies the base by itself. |
| Clock-Hand Angles
[M] |
Track both hands from 12. |
| Counting Coin Combinations
[M] |
Fix the big coins, then count the rest. |
| Solving Two-Step Equations
[H] |
Subtract the constant, then divide by the coefficient. |
| Linear Inequalities
[H] |
Solve like an equation, then take the largest integer that fits. |
| Combining Like Terms
[H] |
Add the coefficients of matching variable terms. |
| Evaluating Two-Variable Expressions
[H] |
Substitute each value, then follow the order of operations. |
| Systems of Two Equations
[H] |
Eliminate one variable to solve for the other. |
| Systems: Combined Values
[H] |
Solve the system, then form the requested combination. |
| Difference of Two Squares
[H] |
Factor a² − b² into (a − b)(a + b) for a fast computation. |
| Absolute-Value Equations
[H] |
Two solutions sit symmetrically around the center. |
| Consecutive Even Integers
[H] |
The middle term is the sum divided by three. |
| Sum, Product and Squares
[H] |
x² + y² = (x + y)² − 2xy. |
| Chained Substitution
[H] |
Work from the known value up through the chain. |
| Rearranging a Formula
[H] |
Solve the perimeter formula for the unknown side. |
| Equations with a Fraction
[H] |
Clear the denominator first by multiplying both sides. |
| Finding the Percent
[H] |
Divide the part by the whole and multiply by 100. |
| Reversing a Discount
[H] |
Divide the sale price by (1 − p/100) to recover the original. |
| Successive Percent Increases
[H] |
Percents do not simply add - combine the multipliers. |
| Combining Two Discounts
[H] |
Two discounts stack to less than their sum. |
| Profit Percent
[H] |
Profit percent is profit divided by cost, times 100. |
| Inverse Proportion
[H] |
More workers means proportionally less time - the product stays fixed. |
| Unit Rates
[H] |
Find the price for one, then scale to any amount. |
| Map Scales
[H] |
Multiply the map distance by the scale factor. |
| Ratios and a Known Difference
[H] |
The difference in ratio parts equals the real difference. |
| Mixing Solutions
[H] |
The mixture concentration is a weighted average by volume. |
| Sharing in a Three-Part Ratio
[H] |
Divide the total by the sum of the ratio parts. |
| Counting Divisors
[H] |
Add one to each prime exponent and multiply. |
| Sum of Divisors
[H] |
List the divisors in pairs and add. |
| Largest Prime Factor
[H] |
Strip out small primes until only the largest remains. |
| Divisibility by 9
[H] |
A number is divisible by 9 exactly when its digit sum is. |
| When Cycles Coincide
[H] |
The next shared moment is the least common multiple. |
| Greatest Common Divisor in Action
[H] |
Equal groups with no leftovers means the GCD. |
| Remainders
[H] |
The remainder is what is left after taking out full groups. |
| Simultaneous Remainders
[H] |
Search the numbers with the first remainder for the second condition. |
| Reversing a Two-Digit Number
[H] |
The difference is always 9 times the digit gap. |
| Reading Other Bases
[H] |
Each digit is weighted by a power of the base. |
| Converting to Another Base
[H] |
Divide repeatedly by the base and read remainders upward. |
| Counting Perfect Squares
[H] |
Count the whole numbers whose square lands in the range. |
| The Largest Impossible Amount
[H] |
For coprime a and b, the answer is ab − a − b. |
| Trailing Zeros of a Factorial
[H] |
Count the factors of 5 in the factorial. |
| Digit Sums
[H] |
Add the individual digits of the number. |
| Counting Multiples in a Range
[H] |
Subtract counts of multiples up to each endpoint. |
| Factor Pairs
[H] |
Pair each divisor with its partner. |
| Power of a Power
[H] |
Multiply the exponents. |
| Product of Powers
[H] |
Add the exponents when the base is the same. |
| Negative Exponents
[H] |
A negative exponent means a reciprocal. |
| Scientific Notation
[H] |
The exponent counts places the decimal point moves. |
| Multiplying in Scientific Notation
[H] |
Multiply the fronts, add the exponents, then renormalize. |
| Simplifying Radicals
[H] |
Pull out the largest perfect-square factor. |
| Cube Roots
[H] |
Find the number that cubes to the given value. |
| Estimating Square Roots
[H] |
Find the largest square that does not exceed the number. |
| Comparing Powers
[H] |
Estimate each power's size to rank them. |
| Solving Exponential Equations
[H] |
Rewrite the right side as a power of the same base. |
| Arithmetic nth Term
[H] |
Add the common difference n − 1 times to the first term. |
| Counting Terms in a Sequence
[H] |
Divide the total gap by the step, then add one. |
| Triangular Numbers
[H] |
The nth triangular number is n(n + 1)/2. |
| Fibonacci Numbers
[H] |
Each term is the sum of the two before it. |
| Summing a Geometric Sequence
[H] |
Use a(rⁿ − 1)/(r − 1). |
| Summing Multiples
[H] |
Factor out the common multiple, then sum 1 to n. |
| Sum of Odd Numbers
[H] |
The first n odd numbers add to n². |
| Sum of Even Numbers
[H] |
The first n even numbers add to n(n + 1). |
| Sum of Squares
[H] |
Use n(n + 1)(2n + 1)/6. |
| Filling In a Sequence
[H] |
Find the constant step, then apply it. |
| Angles in a Quadrilateral
[H] |
The four angles add to 360°. |
| Polygon Angle Sum
[H] |
Split the polygon into (n − 2) triangles. |
| Each Angle of a Regular Polygon
[H] |
Divide the total angle sum by the number of angles. |
| Exterior Angles of Regular Polygons
[H] |
The exterior angles always total 360°. |
| Angles with Parallel Lines
[H] |
Same-side interior angles are supplementary. |
| Isosceles Triangle Angles
[H] |
The two base angles are equal and share the leftover. |
| The Exterior Angle Theorem
[H] |
An exterior angle equals the two remote interior angles. |
| Area from Perimeter and a Side
[H] |
Recover the other side, then multiply. |
| Square Area from Perimeter
[H] |
The side is a quarter of the perimeter. |
| Similar Triangles
[H] |
Corresponding sides share one scale factor. |
| Ratio of Areas
[H] |
Area scales with the square of the side ratio. |
| Area of a Sector
[H] |
Take the angle's fraction of the whole circle's area. |
| Arc Length
[H] |
Take the angle's fraction of the circumference. |
| Volume of a Cylinder
[H] |
Volume is the circular base area times the height. |
| Surface Area of a Cylinder
[H] |
Add the two circular ends to the curved side. |
| Surface Area of a Cube
[H] |
Six identical square faces. |
| Surface Area of a Box
[H] |
Three pairs of matching rectangular faces. |
| Volume of a Sphere
[H] |
Volume is four-thirds π r cubed. |
| Volume of a Cone
[H] |
A cone is one-third of the matching cylinder. |
| Distance Between Points
[H] |
Use the horizontal and vertical gaps with the Pythagorean theorem. |
| Midpoint of a Segment
[H] |
Average the endpoints' coordinates. |
| Slope of a Line
[H] |
Slope is rise over run. |
| Reflecting a Point
[H] |
Reflecting over the y-axis negates the x-coordinate. |
| Translating a Point
[H] |
Add the horizontal shift to the x-coordinate. |
| Rotating a Point 90°
[H] |
A 90° counterclockwise turn sends (x, y) to (−y, x). |
| Area of a Trapezoid
[H] |
Average the parallel sides, then multiply by the height. |
| Area of a Parallelogram
[H] |
Base times perpendicular height. |
| Area of a Rhombus
[H] |
Half the product of the diagonals. |
| Area of a Ring
[H] |
Subtract the inner circle's area from the outer. |
| Diagonal of a Rectangle
[H] |
The diagonal is the hypotenuse of the two sides. |
| Space Diagonal of a Box
[H] |
Combine all three edge lengths under one square root. |
| Ladder Against a Wall
[H] |
The wall, ground, and ladder form a right triangle. |
| The Multiplication Principle
[H] |
Multiply the choices at each independent stage. |
| Arranging Distinct Objects
[H] |
The number of orderings is n factorial. |
| Arrangements with Repeated Letters
[H] |
Divide n! by the factorials of each repeat count. |
| Circular Arrangements
[H] |
Fix one seat to remove equivalent rotations. |
| Complementary Probability
[H] |
Subtract the unwanted probability from 1. |
| Basic Probability
[H] |
Favorable outcomes over total outcomes. |
| Drawing Without Replacement
[H] |
Multiply the probabilities, updating after the first draw. |
| Expected Value
[H] |
Average the outcomes weighted by their probabilities. |
| Geometric Probability
[H] |
Probability is the ratio of favorable area to total area. |
| Inclusion-Exclusion Counting
[H] |
Add the two counts, then subtract the overlap. |
| Diagonals of a Polygon
[H] |
Each vertex connects to all but its two neighbors and itself. |
| Counting Subsets
[H] |
Each element is either in or out - that is 2ⁿ. |
| Even Sums and Parity
[H] |
A sum is even only from two evens or two odds. |
| Speed from Distance and Time
[H] |
Speed is distance divided by time. |
| Trains Approaching
[H] |
Add the speeds to get the closing rate. |
| Catching Up
[H] |
The gap closes at the difference of the speeds. |
| Average Speed (Equal Distances)
[H] |
With equal distances, use the harmonic mean of the speeds. |
| Combined Filling Rates
[H] |
Add the individual rates per hour. |
| Boat and Current
[H] |
Still-water speed is the average of downstream and upstream. |
| Weighted Averages
[H] |
Total everything, then divide by the total count. |
| A Changing Average
[H] |
Rebuild the total, add the new value, divide by the new count. |
| Finding the Median
[H] |
Sort the values and take the middle one. |
| The Range of Data
[H] |
Subtract the smallest value from the largest. |
| Mean from a Frequency Table
[H] |
Weight each value by how often it occurs. |
| Average of Consecutive Integers
[H] |
The mean is the average of the first and last. |
| Pie-Chart Angles
[H] |
A percent of the whole is that percent of 360°. |
| Days of the Week
[H] |
Weekdays cycle every 7 days - use the remainder. |
| Elapsed Time
[H] |
Convert both times to minutes and subtract. |
| 24-Hour Clock Arithmetic
[H] |
Hours wrap around every 24 - take the remainder. |
| Working Backward
[H] |
Undo each operation with its inverse, in reverse order. |
| Maximizing a Product
[H] |
For a fixed sum, the product is largest when the numbers are closest. |
| Sharing With Nothing Left Over
[H] |
Decide which candidate total can be shared equally by testing divisibility instead of dividing. |
| The Digit Under the Blot
[H] |
Choose a hidden digit so that the digit sum or the last digit satisfies a divisibility test. |
| Crossing Out One Digit
[H] |
Remove the single digit that leaves behind a digit sum with the required divisibility. |
| Divisibility Hidden in a Product
[H] |
Decide whether a product is divisible by a number by inspecting its factors instead of multiplying. |
| The Total That Cannot Be Made
[H] |
Test which totals can be built from whole packets of two given sizes. |
| Digits With a Given Sum
[H] |
Find the largest or smallest number whose digits add to a stated total. |
| Clues About a Reversed Number
[H] |
Recover a two-digit number from clues about the number with its digits swapped. |
| Hidden Digits in an Addition
[H] |
Recover blotted digits from a written column addition, carrying where needed. |
| Digits With a Given Product
[H] |
Find a number from the product of its digits, sometimes with a second clue. |
| Digits Tied Together by a Rule
[H] |
Use a stated relation between the digits to list the candidates and pick the one that fits. |
| Counting Numbers With a Digit Property
[H] |
Count how many numbers in a range satisfy a condition on their digits. |
| The Bead at a Far Position
[H] |
Locate a position inside a repeating pattern by dividing and reading the remainder. |
| How Many Beads of One Color
[H] |
Count how often an item occurs in a repeating pattern using whole periods plus the leftover part. |
| Finding a Number From Its Remainders
[H] |
Identify a number from the remainders it leaves and the range it lies in. |
| Remainders After Adding or Multiplying
[H] |
Work out the remainder of a sum or product from the remainders of its parts. |
| A Fraction of What Is Left
[H] |
Take a fraction of the remainder rather than of the original whole. |
| The Whole From a Fractional Part
[H] |
Recover the whole quantity when a fraction of it, or the part left over, is known. |
| Which Fraction Is Nearest
[H] |
Compare fractions by measuring their distance from a benchmark such as 1/2 or 1. |
| The Shaded Fraction of a Figure
[H] |
Read a fraction off a tiled figure and use it to answer a further question. |
| How Many Fit the Fractions
[H] |
Use the denominators of a fraction description to pin down the possible size of the whole. |
| Comparing Percent Amounts
[H] |
Work out several percent amounts of different bases and compare them. |
| Two Percent Changes in a Row
[H] |
Apply a second percent change to the amount produced by the first, not to the original. |
| Percent More and Percent Fewer
[H] |
Compare two groups when one is a percent more or a percent fewer than the other. |
| Percents Against Fractions
[H] |
Order amounts written as fractions, percents and decimals by rewriting them in one form. |
| Sharing When the Difference Is Known
[H] |
Size one share from the difference between two parts of a ratio, then answer the question asked. |
| Linking Two Ratios Together
[H] |
Pass through a shared quantity to connect two ratios into one. |
| From a Ratio to a Fraction
[H] |
Convert between a ratio of parts and the fraction each part is of the whole. |
| Replacing One Value in an Average
[H] |
Track how the average moves when a single value is replaced by another. |
| The Score Needed Next
[H] |
Use totals to find the extra value that moves an average to a target. |
| Removing a Value From an Average
[H] |
Compare the totals before and after a value is removed to identify it. |
| Averages of Numbers in a Run
[H] |
Use the symmetry of a run of consecutive numbers to link its average, ends and total. |
| Two Rules Taking Turns
[H] |
Discover a rule in which two different steps alternate, then continue the row. |
| Running a Rule Backwards
[H] |
Undo a repeated rule step by step, reversing each operation in turn. |
| Add the Digit Sum Each Time
[H] |
Follow a rule whose step depends on the digits of the current number. |
| Where a Number Sits in a Row
[H] |
Turn a step rule around to find a position, or the first term past a limit. |
| Rows Where Every Three Add to the Same Total
[H] |
Use a constant sum of neighbouring cells to show that the row repeats every three places. |
| Putting In Plus and Minus Signs
[H] |
Decide which totals can be reached by choosing plus or minus in front of each number. |
| Where the Brackets Go
[H] |
Compare the values an expression takes under every possible single bracketing. |
| Arranging Digit Cards for the Best Result
[H] |
Place digits into two numbers so that their sum, product or difference is extreme. |
| The Order of Two Machines
[H] |
Compare the results of applying two operations in either order, and undo them. |
| Finding the Hidden Operations
[H] |
Choose the operations that make a number sentence true, respecting the order of operations. |
| Counting Heads and Legs
[H] |
Split a group of two kinds of creature using the head count and the leg count. |
| Two or Three Numbers From Their Sums
[H] |
Find individual amounts from totals and differences by combining the given equations. |
| Two Kinds of Item, One Bill
[H] |
Compare two shopping bills so that one item cancels and the other is revealed. |
| Passing Some Over
[H] |
Track how a transfer changes two amounts, remembering that one loses exactly what the other gains. |
| A Journey in Stages
[H] |
Choose the smallest stage as the unknown and write every other stage in terms of it. |
| Same Fence, Different Field
[H] |
Compare the areas of rectangles that share one perimeter, and find the extreme cases. |
| Shortest Fence for a Given Area
[H] |
List the whole-number side pairs that give an area and pick the extreme perimeter. |
| Cutting a Rectangle into Equal Strips
[H] |
Find the perimeter of one strip, or of all of them, after a rectangle is cut into equal strips. |
| The Path Around the Pond
[H] |
Find the area of a border of constant width by subtracting the inner rectangle from the outer one. |
| Rectangles Built from Identical Tiles
[H] |
Work out the size of a rectangle assembled from identical tiles, and the fewest tiles that make a square. |
| The Perimeter of a Staircase
[H] |
Slide the steps of a staircase outwards to see that its perimeter equals that of the surrounding rectangle. |
| The Missing Side of an L-Shape
[H] |
Use the fact that opposite sides of an L-shape add up in pairs to find a missing length or the perimeter. |
| Two Rugs That Overlap
[H] |
Add the two areas and subtract the overlap once, because the overlap has been counted twice. |
| The Perimeter of a Square Tile Shape
[H] |
Count the exposed unit edges of a shape built from squares instead of adding side lengths. |
| A Rectangle with a Hole
[H] |
Subtract the hole for area, but add the hole's edge for the total length of edge. |
| A Triangle Inside a Rectangle
[H] |
Use base times height divided by two, noticing that the tip may sit anywhere on the opposite side. |
| What Fraction Is Shaded
[H] |
Express a shaded part of a figure as a fraction of the whole in lowest terms. |
| The Area of a Tilted Shape
[H] |
Find the area of a tilted lattice figure by subtracting the corner triangles from the surrounding rectangle. |
| Shaded Against White
[H] |
Compare a shaded region with the rest of a figure by counting or by subtracting from the whole. |
| Three Angles on a Straight Line
[H] |
Use the fact that angles sitting side by side on a straight line total 180 degrees. |
| Angles Filling a Whole Turn
[H] |
Share 360 degrees among angles that meet at a point, including equal shares and shares in a ratio. |
| A Line Drawn Across a Triangle
[H] |
Chase angles through a triangle cut by a line from one vertex to the opposite side. |
| Turning Round and Round
[H] |
Track a direction through repeated turns, remembering that a full turn is 360 degrees. |
| Slices of a Round Cake
[H] |
Convert between the center angle of a slice and the fraction of the circle it takes up. |
| Angles Made by Folding Paper
[H] |
Use the fact that a fold reflects an angle, creating two equal angles at the fold line. |
| Which Net Folds into a Cube
[H] |
Decide whether six joined squares fold into a closed cube by checking that no two squares land on the same face. |
| Opposite Faces on a Net
[H] |
Identify which squares of a net become opposite faces once the net is folded up. |
| From the Flat Net to the Box
[H] |
Recover a box's dimensions from the rectangles of its net, then find its volume or its card area. |
| Where the Sixth Square Can Go
[H] |
Test each free position for a sixth square by checking whether the completed shape folds into a cube. |
| Numbers on the Net of a Die
[H] |
Combine the opposite-face rule with the folding of a net to find a missing or extreme face total. |
| Counting the Squares in a View
[H] |
Count the squares in a view of a cube stack by taking the tallest pile in each line of sight. |
| Which View Shows the Most
[H] |
Work out all three views of a cube stack and compare how many squares each one shows. |
| What the Stack Looks Like from There
[H] |
Read off the bar heights of a view, in the correct left-to-right order for the direction of viewing. |
| Fewest and Most Cubes for Two Views
[H] |
Find the least or the greatest number of cubes that can produce a given front view and side view. |
| The Cubes You Cannot See
[H] |
Count the cubes of a block that have no face on any visible surface, remembering that the floor hides one layer. |
| Filling the Box with Cubes
[H] |
Subtract the cubes already placed from the capacity of a box, or from the smallest cube that contains a block. |
| Reading a Plan of Piles
[H] |
Use a top view labeled with pile heights to count the cubes, the visible tops, or the cubes still missing. |
| Glued Faces and Surface
[H] |
Relate the surface of a solid made of unit cubes to the number of glued face-to-face joins. |
| Turning a Pattern onto Itself
[H] |
Measure the rotational symmetry of a pattern by counting the turns that leave it unchanged. |
| Shading to Make It Symmetric
[H] |
Add the fewest squares needed for a pattern to gain a mirror line or half-turn symmetry. |
| How Many Mirror Lines
[H] |
Test each candidate line of a grid pattern by checking whether every shaded square has a shaded partner. |
| The Shape After a Turn
[H] |
Predict how a shape drawn on a grid looks after a quarter turn, a half turn, or a reflection. |
| Covering a Floor with Tiles
[H] |
Count the tiles that cover a rectangle exactly, and decide when an exact covering is possible at all. |
| Covering a Board with Dominoes
[H] |
Use area and the black-and-white coloring of a board to decide whether dominoes can cover it. |
| Square Tiles That Fit Exactly
[H] |
Cover a rectangle with equal square tiles by using the greatest common divisor of its sides. |
| The Shortest Walk Through the Streets
[H] |
Find the length of a shortest route along grid streets, including detours forced by closed crossings. |
| Counting Routes Past a Closed Crossing
[H] |
Count shortest grid routes when some crossings are closed or one crossing must be used. |
| An Ant Walking on a Box
[H] |
Measure routes along the edges of a box, from a single crossing to a tour of every corner. |
| Drawing a Figure in One Stroke
[H] |
Count the corners where an odd number of lines meet to decide whether a figure can be drawn in one stroke. |
| Cutting into Identical Pieces
[H] |
Divide a figure's area by the number of identical pieces, and scale lengths when the pieces are small copies. |
| Cut Up and Put Together Again
[H] |
Use the fact that cutting and rearranging changes the shape but never the total area. |
| Lining Up When One Place Is Fixed
[H] |
Count the orders of a line when one person is tied to an end place, by ordering only the free places. |
| Neighbours Who Must, or Must Not, Be Together
[H] |
Treat a group that must stay together as one block, and subtract the together count to get the apart count. |
| Seats Around a Round Table
[H] |
Count circular seatings by holding one place fixed, then halve the count when mirror images also match. |
| Coloring Stripes with No Two Neighbours Alike
[H] |
Color cells one at a time, counting the choices left for each cell once its neighbours are fixed. |
| Choosing a Pair When Order Does Not Matter
[H] |
Count unordered pairs as n(n-1)/2, then adjust for a banned pair or for a pair that must include one girl. |
| Choosing Three from a Small Group
[H] |
Count unordered triples as n(n-1)(n-2)/6, then adjust for a member who is fixed or a pair that is banned. |
| Numbers Built from Given Digits
[H] |
Count numbers made from a digit set by filling the most restricted place first, such as the last digit or the leading digit. |
| Choices with One Combination Banned
[H] |
Multiply the free choices, then subtract the combinations a rule forbids, adding back any removed twice. |
| Rearranging Letters When Some Repeat
[H] |
Divide the count of all orders by the orders of each repeated letter, since swapping identical letters changes nothing. |
| How Many Different Selections
[H] |
Count selections from n kinds as 2 to the power n, one yes-or-no decision per kind, then remove the selections a rule forbids. |
| Routes When a Crossing Is Closed
[H] |
Count shortest routes on a street grid by writing on each crossing the number of routes that reach it, entering 0 at a closed crossing. |
| Following the One-Way Roads
[H] |
Count journeys through a one-way network by labeling each town with the number of ways to reach it from the start. |
| Climbing Steps One or Two at a Time
[H] |
Count the ways to reach each step as the sum of the ways to reach the steps a single hop below it. |
| Drawing a Figure in One Stroke
[H] |
Count the points where an odd number of lines meet: half that number is the smallest number of strokes needed. |
| Routes That Must Pass a Given Crossing
[H] |
Split a route at a compulsory crossing and multiply the routes of the two halves, adding the two cases when either of two crossings will do. |
| Games in an All-Play-All Tournament
[H] |
Count the games of an all-play-all tournament as the number of pairs of teams, doubling it when every pair meets twice. |
| Finding the Number of Teams from the Games
[H] |
Work backwards from the number of games to the number of teams by finding which consecutive pair of numbers has the right product. |
| Matches in a Knockout Tournament
[H] |
Count knockout games by counting the losses needed, since every game produces exactly one loss and every player but the champion is knocked out. |
| Reading a Points Table
[H] |
Turn wins, draws and losses into points with 3 and 1, and work backwards from a points total to the number of wins or draws. |
| Games Still to Be Played
[H] |
Halve the total of the games-played column, because each game is counted once by each of the two teams in it, then subtract from the full fixture list. |
| When Exactly One Statement Is True
[H] |
Test each offered number against every statement and keep the one whose number of true statements matches the rule. |
| How Many of Them Are Lying
[H] |
Use the fact that all truthful speakers must have said the same number, so the count of speakers claiming a value has to match that value. |
| Who Broke the Window
[H] |
Suppose each suspect in turn is guilty, count how many statements that makes true, and keep the suspect whose count fits. |
| Knights and Liars Talking About Each Other
[H] |
Pass the known type along the chain: a speaker calling someone a liar is of the opposite type when the claim is true. |
| Finding the Light Coin on a Balance
[H] |
Split the coins into three equal piles so that one weighing cuts the search to a third, giving three to the power of the weighings. |
| Matching People from a Table of Clues
[H] |
Cross out the impossible squares of a matching grid until each row and each column has exactly one square left. |
| Houses in a Row
[H] |
Place the most restricted clue first, such as a pair that must be side by side, and slide it along the row until every clue fits. |
| Ordering People from Comparisons
[H] |
Build one line from the comparisons by inserting each person to the correct side of those already placed. |
| The Finishing Order of a Race
[H] |
Turn each clue into a gap between finishing places, then fit the fixed blocks into the five places. |
| Balance Scales and Swapping Shapes
[H] |
Reduce each balance to the value of one shape, then swap shapes for their equals until only the wanted shape is left. |
| Who Sits Where at the Round Table
[H] |
Fix one person to remove the turning, then walk round the table placing each neighbour clue in the seats that remain. |
| Socks in the Dark
[H] |
Find the largest unlucky handful that still fails, then add one sock to force the wanted outcome. |
| Making Sure of Several of One Color
[H] |
Add up how many of each color can be taken while staying one short of the target, then take one more. |
| Making Sure of One of Every Kind
[H] |
Take everything except the scarcest kind for the worst case, then add one so that kind must appear. |
| How Many People Before Two Must Share
[H] |
Treat each category as a box: one more person than there are boxes forces two into one box. |
| The Fullest Box and the Emptiest Box
[H] |
Share the objects out as evenly as the rules allow, since the fullest box can never be below the average and the emptiest can never be above it. |
| Worst Case with Gloves and Tickets
[H] |
Describe the largest handful that still fails, choosing the worse side of every pairing, and then add one. |
| Probability After Some Are Taken Away
[H] |
Recount the favourable beads and the total beads after the change, then read the probability as one count over the other. |
| Two Beads Drawn Without Replacing
[H] |
Multiply the chance of the first draw by the chance of the second when one bead has already gone, and add the separate color cases. |
| Two Spins of a Spinner
[H] |
List the ordered pairs of results as the total count, then count the pairs that satisfy the condition. |
| Two Dice and a Condition
[H] |
Work over the thirty-six ordered outcomes of two dice and count the ones that meet the condition. |
| At Least One, Counted the Short Way
[H] |
Find the probability of at least one success by taking one minus the probability that every trial fails. |
| The Chance That a Random Line-Up Works
[H] |
Divide the number of arrangements that satisfy the condition by the total number of arrangements. |
| Counting the Ones Left Out
[H] |
Add the group totals, subtract each overlap once and add the triple overlap back, then subtract from the whole class. |
| Choosing Seats with No Two Together
[H] |
Seat the chosen children first and then slot the empty chairs into the gaps, which turns the problem into a plain selection. |
| How Many Numbers Contain a Given Digit
[H] |
Count the numbers whose tens digit matches and those whose units digit matches, then subtract the ones counted twice. |