293 core topics
· approximately 87 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.
| Tens and Ones
[E] |
A two-digit number is some tens and some ones. |
| Building Two-Digit Numbers
[E] |
Put tens and ones together to make a number. |
| 10 More and 10 Less
[E] |
Change the tens digit by one. |
| Counting by Tens
[E] |
Skip-count 10, 20, 30, and so on. |
| Adding Tens
[E] |
Add a multiple of ten to a two-digit number. |
| Comparing Two-Digit Numbers
[M] |
Use <, >, or = to compare. |
| Telling Time to the Hour
[E] |
Read a clock on the hour. |
| Telling Time to the Half-Hour
[E] |
Read a clock on the half-hour. |
| Comparing Lengths
[M] |
Order objects by length using clues. |
| Measuring Length in Units
[E] |
Count how many whole units long an object is. |
| Putting Events in Order
[E] |
Decide what comes first, next, and last. |
| Count to 120
[E] |
Count on past 100, all the way to 120. |
| Subtracting Tens
[E] |
Take away 10, 20, 30 from a multiple of ten. |
| Measuring with Non-Standard Units
[E] |
Lay same-size objects end to end and count them. |
| Add Within 20
[M] |
Add two numbers with a total of 20 or less. |
| Subtract Within 20
[M] |
Take away to find how many are left, within 20. |
| Doubles to 20
[M] |
Add a number to itself, up to 10 + 10. |
| Near Doubles
[M] |
Add two numbers that are one apart using a double. |
| Adding by Making a Ten
[M] |
Break a number apart to make a friendly ten first. |
| Missing Addends
[M] |
Find the number that completes an addition. |
| Add Three Numbers
[M] |
Add three numbers by looking for a friendly pair. |
| Fact Families
[M] |
Use a known addition to find a subtraction. |
| Add Ten to a Number
[M] |
Add one whole ten to a two-digit number. |
| Add Multiples of Ten
[M] |
Add tens like you add ones. |
| Count to 120
[M] |
Keep counting past 100 all the way to 120. |
| Count by Tens to 120
[M] |
Skip-count by tens up toward 120. |
| How Many Tens
[M] |
Read the tens digit of a two-digit number. |
| How Many Ones
[M] |
Read the ones digit of a two-digit number. |
| Value of the Tens Digit
[M] |
Tell the value the tens digit stands for. |
| Build a Number from Tens and Ones
[M] |
Join tens and ones to make a two-digit number. |
| Ten More, Ten Less
[M] |
Jump a whole ten up or down from a number. |
| Compare Two-Digit Numbers
[M] |
Decide which two-digit number is greater. |
| Greater Than, Less Than, Equal
[M] |
Choose the >, <, or = symbol that compares two numbers. |
| Order Three Numbers
[M] |
Find the smallest of three two-digit numbers. |
| Tell Time to the Hour
[M] |
Read a clock when the minute hand is on 12. |
| Tell Time to the Half-Hour
[M] |
Understand what half past means on a clock. |
| Hours Later
[M] |
Find the time a few whole hours later. |
| Count Nickels
[M] |
Find the value of a handful of nickels. |
| Count Dimes
[M] |
Find the value of a handful of dimes. |
| Count Mixed Coins
[M] |
Add up the value of nickels and pennies together. |
| Measure Length with Units
[M] |
Measure how long something is using same-size units. |
| Compare Two Lengths
[M] |
Decide which of two objects is longer. |
| Order Objects by Length
[M] |
Find the shortest of three objects. |
| Combine Lengths
[M] |
Add two lengths placed end to end. |
| Defining Shape Attributes
[M] |
Tell which features truly define a shape. |
| Compose Shapes
[M] |
Combine smaller shapes to build a bigger one. |
| Halves
[M] |
Understand cutting a whole into two equal parts. |
| Fourths (Quarters)
[M] |
Understand cutting a whole into four equal parts. |
| Half of a Number
[M] |
Split an even number into two equal parts. |
| Count Equal Shares
[M] |
Match the name of a share to how many parts. |
| Skip Count by Twos
[M] |
Count 2, 4, 6, 8 and keep the pattern going. |
| Skip Count by Fives
[M] |
Count 5, 10, 15, 20 and keep the pattern going. |
| Compare Word Problems
[M] |
Find how many more one person has than another. |
| Two-Step Word Problems
[M] |
Solve a story with two changes in a row. |
| Counting Back to Subtract
[M] |
Subtract a small number by counting back from the bigger one. |
| Doubles Minus One
[M] |
Add two numbers one apart using the higher double. |
| Making a Ten to Subtract
[M] |
Subtract across ten by stopping at ten on the way. |
| Order Does Not Change the Sum
[M] |
Use the fact that addends can be added in any order. |
| Ten-Plus Facts
[M] |
Add a one-digit number to ten to make a teen. |
| Expanded Form
[M] |
Break a two-digit number into its tens and ones values. |
| Two-Digit Number Words
[M] |
Match a two-digit number word to its numeral. |
| One More, One Less to 120
[M] |
Add or take away one anywhere up to 120. |
| Numbers In Between
[M] |
Name the two-digit number between two others. |
| Digital Clock Times
[M] |
Match an o'clock or half-past time to a digital clock. |
| Count Quarters
[M] |
Find the value of a few quarters. |
| Count Dimes and Pennies
[M] |
Add the value of dimes and pennies together. |
| Count by Twenty-Fives
[M] |
Skip-count 25, 50, 75, 100 like counting quarters. |
| How Many More Cents
[M] |
Find how many more cents are needed to reach a price. |
| Build with Solid Shapes
[M] |
Combine solid shapes to make a new solid. |
| Equal and Unequal Shares
[M] |
Know that fair shares must be equal in size. |
| Thirds
[M] |
Understand splitting a whole into three equal parts. |
| Bigger: a Half or a Fourth
[M] |
Compare the size of a half to a fourth. |
| A Fourth of a Number
[M] |
Split a number into four equal groups. |
| Name the Shaded Fraction
[M] |
Say the fraction name for one shaded equal part. |
| Parts That Make a Whole
[M] |
Count how many equal parts rebuild the whole. |
| How Many Fewer
[M] |
Find how many fewer one group has than another. |
| Read a Bar Graph
[M] |
Read the value of one bar on a bar graph. |
| Which Category Has the Fewest
[M] |
Read three groups and find the smallest. |
| Add-To: Change Unknown
[M] |
Find how many were added when the total is known. |
| Take-From: Start Unknown
[M] |
Find the starting amount when some were taken away. |
| Put-Together: Missing Part
[M] |
Find one part when the whole and the other part are known. |
| Greatest of Three Numbers
[M] |
Find the biggest of three two-digit numbers. |
| How Much Longer
[M] |
Find how much longer one object is than another. |
| Two-Digit Minus One-Digit
[M] |
Subtract a one-digit number from the ones place. |
| True or False Subtraction
[M] |
Decide which subtraction sentence is correct. |
| Count On from the Bigger Number
[H] |
Add by starting at the larger addend and counting on the smaller one. |
| Count On with Hops on a Number Line
[H] |
Add on a number line by hopping forward one space for each one added. |
| Count Back with Hops on a Number Line
[H] |
Subtract on a number line by hopping back one space for each one taken. |
| Count Up to Find a Difference
[H] |
Find a difference by counting up from the smaller number to the larger. |
| Plus One, Plus Two, Plus Three
[H] |
Add one, two or three to a number by naming the next counting numbers. |
| Partners That Make Ten
[H] |
Name the number that pairs with a given number to make ten. |
| Break Apart to Make a Ten
[H] |
Split the second addend so the first one grows into a ten. |
| Add Nine by Adding Ten
[H] |
Add nine by adding ten and then taking one away. |
| Take Ten from a Teen
[H] |
Subtract ten from a teen number to leave the ones. |
| Doubles Plus Two
[H] |
Add two numbers that are two apart by using the smaller double. |
| Choose the Helping Double
[H] |
Pick the doubles fact that helps with a near-doubles sum. |
| Doubles in an Array
[H] |
Read a double from two equal rows of dots and total them. |
| Which Number Was Doubled
[H] |
Find the number that was added to itself to give a total. |
| Use a Known Fact: One More
[H] |
Find a new sum from a known sum when one addend grows by one. |
| Use a Known Fact: One Less
[H] |
Find a new sum from a known sum when one addend shrinks by one. |
| The Matching Subtraction Fact
[H] |
Write the true subtraction sentence that uses the numbers of a known sum. |
| Pairs with the Same Total
[H] |
Recognize two addition pairs that have equal totals. |
| Give and Take to Add
[H] |
Move ones from one addend to the other so the sum becomes ten plus ones. |
| Subtract by Thinking Addition
[H] |
Solve a subtraction by finding the addend that completes the whole. |
| Check a Subtraction by Adding
[H] |
Test a subtraction answer by adding it back to the number taken away. |
| Check an Addition by Subtracting
[H] |
Test an addition answer by subtracting one addend from the total. |
| Take-From: How Many Were Taken
[H] |
Find how many were taken away when the start and the rest are known. |
| Add Three: Make a Ten First
[H] |
Add three numbers by pairing the two that make ten first. |
| Add Three: Use a Double First
[H] |
Add three numbers by pairing the two equal ones first. |
| Three Numbers with One Missing
[H] |
Find the third addend when three numbers and their total are given. |
| Choose the Easiest Pair First
[H] |
Decide which two of three addends to add first to make the work easy. |
| Unknown in the First Place
[H] |
Find the first addend when the second addend and the total are known. |
| Unknown at the Start of a Subtraction
[H] |
Find the starting number when the amount taken and the rest are known. |
| Unknown Amount Taken Away
[H] |
Find how much was subtracted when the start and the result are known. |
| Match the Story to the Equation
[H] |
Choose the number sentence whose unknown sits where the story needs it. |
| Balance Both Sides
[H] |
Make an equation true when both sides are additions. |
| Balance with a Subtraction
[H] |
Make an equation true when one side is a subtraction. |
| True or False on Both Sides
[H] |
Judge whether two additions on either side of the equal sign match. |
| The Total Written First
[H] |
Solve an equation whose total is written before the equal sign. |
| True or False with Three Numbers
[H] |
Judge whether a three-addend sentence gives the correct total. |
| Find the Number That Makes It True
[H] |
Fill an unknown on the left so the two sides of an equation match. |
| Which Sign Makes It True
[H] |
Choose the plus or minus sign that makes a number sentence true. |
| Compare: How Many More
[H] |
Find the difference between two amounts shown side by side. |
| Compare: Find the Bigger Amount
[H] |
Find the larger amount when one group has a stated number more. |
| Compare: Find the Smaller Amount
[H] |
Find the smaller amount when the larger group has a stated number more. |
| Fewer Than: Find the Bigger Amount
[H] |
Find the larger amount when the smaller group is a stated number fewer. |
| Two-Step: Put Together Twice
[H] |
Combine two groups and then join a third to find a total. |
| Two-Step: Some Join, Some Leave
[H] |
Work through a story where a group first grows and then shrinks. |
| Two-Step: Take Away Twice
[H] |
Work through a story where two amounts are taken away one after another. |
| Two-Step: Compare Then Total
[H] |
Find one amount from a comparison, then add both amounts together. |
| Two-Step: Find the Hidden Change
[H] |
Find an unknown change that comes after a known first step. |
| Count Forward to 120
[H] |
Keep counting forward by ones from any number up to 120. |
| Missing Number on a Number Chart
[H] |
Fill a blank square in a number chart using one more and ten more. |
| Count a Big Group by Tens
[H] |
Count equal bags of ten and any loose ones to find a total. |
| Number Words Past One Hundred
[H] |
Match a number word between 101 and 120 to its numeral. |
| Count Backward from 120
[H] |
Count backward by ones from numbers up to 120. |
| Read Base-Ten Blocks
[H] |
Read a two-digit number from a picture of ten-rods and single cubes. |
| Extra Ones Make Another Ten
[H] |
Rebuild a number when the ones are ten or more. |
| Trade a Ten for Ten Ones
[H] |
Show the same number with fewer tens and more ones. |
| Ten Tens Make One Hundred
[H] |
Count tens past one hundred, where ten tens make a hundred. |
| Jumps of Ten on a Number Line
[H] |
Land on the right number after jumps of ten on a number line. |
| Ten More Past One Hundred
[H] |
Find ten more or ten less for numbers near and past one hundred. |
| How Big Was the Jump
[H] |
Decide whether a number changed by one or by ten, and by how many tens. |
| Ten More in a Story
[H] |
Solve a story problem where an amount grows or shrinks by ten. |
| Compare Numbers Shown with Blocks
[H] |
Compare two two-digit numbers shown as tens and ones. |
| Same Tens, Compare the Ones
[H] |
Compare two-digit numbers that have the same tens digit. |
| Compare a Number to a Sum
[H] |
Work out a sum of tens and ones, then compare it with another number. |
| Which Comparison Is True
[H] |
Check comparison sentences and pick the true one. |
| Add Ones and Make a New Ten
[H] |
Add a one-digit number when the ones make a new ten. |
| How Many More to the Next Ten
[H] |
Find how many ones are needed to reach the next ten. |
| Missing One-Digit Addend
[H] |
Find the missing one-digit addend in a two-digit addition. |
| Two-Digit Plus One-Digit Story
[H] |
Solve a story problem that adds ones to a two-digit number. |
| The Tens Digit After Adding
[H] |
Decide how many tens the answer has after adding ones. |
| How Many Tens in All
[H] |
Add groups of tens and say the answer as a number of tens. |
| Add Tens to a Two-Digit Number
[H] |
Add a multiple of ten to a two-digit number without changing the ones. |
| How Many Tens Were Added
[H] |
Find the multiple of ten that was added to reach a total. |
| Adding Tens from a Graph
[H] |
Read two multiples of ten from a bar graph and add them. |
| Subtract Tens from a Two-Digit Number
[H] |
Take a multiple of ten away from a two-digit number. |
| How Many Tens Were Taken
[H] |
Find the multiple of ten that was subtracted. |
| Taking Away Tens in a Story
[H] |
Solve a story problem where whole tens are given away. |
| Fact Families with Tens
[H] |
Match addition and subtraction facts made from the same three multiples of ten. |
| Compare Lengths Without Measuring
[H] |
Compare two objects using a third object they were both compared to. |
| Rules for Fair Measuring
[H] |
Apply the rules that make a length measurement fair. |
| Smaller Units, Bigger Number
[H] |
Relate the size of the measuring unit to the size of the count. |
| Order Ribbons by Length in a Picture
[H] |
Order three drawn objects by how many length units each one covers. |
| Measure from a Cube Picture
[H] |
Count same-size units in a drawing to measure and compare lengths. |
| Where the Hour Hand Points
[H] |
Read the hour hand at an o'clock time and at a half past time. |
| What the Minute Hand Tells
[H] |
Use the minute hand to tell an o'clock time from a half past time. |
| Half an Hour Later
[H] |
Find the time a half hour or an hour and a half after a given time. |
| Before or After a Given Time
[H] |
Decide whether a clock time comes before or after another time. |
| How Many Half Hours
[H] |
Count how many half hours fit inside a number of hours. |
| How Many in All Three Groups
[H] |
Add every category in a graph to find the total. |
| Put Two Categories Together
[H] |
Add two chosen categories from a three-category graph. |
| Find the Missing Bar
[H] |
Use the total and the bars shown to find a missing category. |
| Most and Fewest: the Difference
[H] |
Find the difference between the largest and smallest categories in a graph. |
| Sort a List into Categories
[H] |
Organize a list of choices into categories and count them. |
| What Does Not Change a Shape
[H] |
Separate the attributes that define a shape from those that do not. |
| Name the Shape from Its Clues
[H] |
Name a flat shape from the number of sides and corners described. |
| Faces and Corners of Solid Shapes
[H] |
Count the flat faces or corners of a common solid shape. |
| Which Solids Roll and Stack
[H] |
Tell which solids roll and which stack, using their flat and curved parts. |
| How Many Triangles Fill the Shape
[H] |
Count the smaller shapes needed to fill several copies of a bigger shape. |
| Cutting a Shape Makes New Shapes
[H] |
Name the shapes made when a flat shape is cut with a straight line. |
| Equal Parts in More Than One Whole
[H] |
Count halves or fourths across several wholes. |
| How Big Was the Hop
[H] |
Counting from the start to the landing place gives the size of the hop. |
| Where the Hop Began
[H] |
To find where a hop began, undo it: hop the same size the other way. |
| The Missing Number on the Line
[H] |
Find the counting rule from the labels, then use it on the gap. |
| Equal Hops on the Line
[H] |
Equal hops skip count, so the landing place and the hop count match up. |
| Empty Boxes in a Ten-Frame
[H] |
Counters and empty boxes in a ten-frame always add up to ten. |
| Putting More Counters in the Frame
[H] |
Count on from the counters shown; past ten the total becomes a teen. |
| Taking Counters Out of the Frame
[H] |
Taking counters out leaves fewer counters and more empty boxes. |
| A Full Frame and Some More
[H] |
A full frame is one ten, so a teen number is ten and the extra ones. |
| Ten and Some More
[H] |
Filling the frame rewrites a hard sum as an easy ten-plus fact. |
| Landing on Ten First
[H] |
Subtract from a teen in two easy hops: back to ten, then the rest. |
| Nothing Added, Everything Taken
[H] |
Adding zero changes nothing, and taking a number from itself leaves zero. |
| Which Card Is Bigger
[H] |
Work out each total first, then compare the totals. |
| Two More, Two Less
[H] |
Change one addend by two and the total changes by two the same way. |
| One More Here, One Less There
[H] |
Moving one from an addend to the other keeps the total the same. |
| The Same Fact, Ten Bigger
[H] |
Adding ten to an addend adds ten to the total, and the ones stay put. |
| Which Fact Helps Most
[H] |
Pick a helper fact that is close to the sum and easy to say. |
| Keeping Both Sides Equal
[H] |
The equal sign says the two sides are worth the same amount. |
| Take-Away on Both Sides
[H] |
Work out the side you can, then make the other side match it. |
| True or False with Tens
[H] |
Adding tens to tens counts tens, so only the tens digit changes. |
| Another Name for a Number
[H] |
One number has many names: sums and take-aways can all mean the same. |
| How Many More to Fill the Order
[H] |
A missing addend is the gap between what you have and what you want. |
| How Many More to Twenty
[H] |
Count on from where you are to twenty to find what is still missing. |
| Filling In a Part-Whole Table
[H] |
The parts add up to the whole, so any one of them can be found. |
| Two Parts That Fit the Clue
[H] |
Check every pair against both the total and the clue before choosing. |
| Three Groups in a Picture
[H] |
Read each row of the picture, then join only the rows the question asks for. |
| Adding Four or Five Numbers
[H] |
Add a long line of numbers two at a time, keeping a running total. |
| Which Numbers Make the Total
[H] |
Test each list by adding it, and keep the one that lands on the total. |
| Fewer Than: Find the Smaller Amount
[H] |
"Fewer than" names the smaller amount, so take the gap away. |
| Add-To: How Many Were There at First
[H] |
When things were added, work backwards from the end to the start. |
| Making Two Amounts Match
[H] |
The gap tells how many to add on; sharing the gap fairly halves it. |
| The Number You Do Not Need
[H] |
Use only the numbers that answer the question asked. |
| Comparing Two Rows of a Table
[H] |
Find the two rows the question names, then compare only those. |
| Is That Enough?
[H] |
Compare what you have with what you need, then say the gap. |
| Finishing a Base-Ten Picture
[H] |
Match the blocks to the digits: tens blocks to the tens, ones to the ones. |
| Swapping the Two Digits
[H] |
Swapping the digits swaps their jobs, so the number changes a lot. |
| Number Riddles with Tens and Ones
[H] |
Work out each digit from its clue, then write the tens digit first. |
| How Many Ten-Frames Does It Take
[H] |
The tens digit counts full frames; the ones digit is what is left over. |
| Jumping Ten More Than Once
[H] |
Each jump of ten changes only the tens digit, so count the jumps. |
| Ten More or Ten Less
[H] |
Ten more moves the tens digit; one more moves the ones digit. |
| Adding Tens to a Block Picture
[H] |
One more tens block is ten more, so only the tens digit moves. |
| Which Digit Changes
[H] |
Adding a multiple of ten moves the tens digit and leaves the ones alone. |
| Tens First, Then Ones
[H] |
Add the tens to the tens first, then deal with the ones. |
| Which One Is the Longest
[H] |
The longest object is the one with the most cubes in its row. |
| Which One Is the Shortest
[H] |
The shortest object is the one with the fewest cubes in its row. |
| The One in the Middle
[H] |
Ordering three lengths leaves one object between the other two. |
| Longer and Shorter from Clues
[H] |
If A beats B and B beats C, then A beats C without any measuring. |
| Measuring with Cubes in a Row
[H] |
Lay same-size cubes end to end and count them to measure a length. |
| The Gap Between Two Cube Rows
[H] |
Line up two rows of cubes and count the extra cubes on the longer one. |
| Joining Lengths End to End
[H] |
Two lengths joined end to end add; a missing piece is found by taking away. |
| Longer and Shorter Stories
[H] |
Shorter means take the gap away; longer means add it on. |
| Does It Fit Inside
[H] |
One thing fits inside another only when it is the shorter one. |
| O'Clock on a Clock Face
[H] |
On the hour the long hand points at 12 and the short hand names the hour. |
| Half Past on a Clock Face
[H] |
At half past the long hand points at 6 and the short hand has just passed the hour. |
| Whole Hours Later
[H] |
One hour later moves the short hand on by one and leaves the long hand alone. |
| Half an Hour On from Now
[H] |
Half an hour later turns o'clock into half past, and half past into the next o'clock. |
| Saying a Time Two Ways
[H] |
The same time can be said in words or written with two dots. |
| Putting Times in Order
[H] |
Earlier times have smaller hour numbers, and o'clock comes before half past. |
| Counting Whole Hours
[H] |
Count the hours from one o'clock time up to another. |
| What Each Coin Is Worth
[H] |
A penny is 1 cent, a nickel is 5 cents and a dime is 10 cents. |
| Adding Up a Purse of Coins
[H] |
Count the dimes by tens, the nickels by fives, then add the pennies. |
| How Many Coins Make the Amount
[H] |
Skip count by the coin's value until you reach the amount. |
| Who Has More Money
[H] |
More coins is not always more money, so work out each total first. |
| Is That Enough Money
[H] |
Add the coins up, then compare that total with the price. |
| Cents Left After Spending
[H] |
Total the coins first, then take away what was spent. |
| Naming the Shape You See
[H] |
A flat shape is named by how many straight sides it has. |
| Counting the Sides
[H] |
A side is one straight edge, counted once all the way round the shape. |
| Counting the Corners
[H] |
A corner is where two sides meet, and a flat shape has as many corners as sides. |
| Which Shape Has That Many Sides
[H] |
Going from a side count back to a shape name is the same list read backwards. |
| What Really Makes the Shape
[H] |
Sides and corners decide a shape's name; color, size and turning do not. |
| Making a New Shape from Shapes
[H] |
Shapes pushed together with no gaps make one new, bigger shape. |
| Naming a Solid Shape
[H] |
Solid shapes are named by their flat faces and their curved parts. |
| Solids All Around Us
[H] |
Everyday objects match the solid shapes: balls, cans, hats and boxes. |
| The Flat Face of a Solid
[H] |
The flat face of a solid is itself a flat shape you can name. |
| Sorting Shapes by Their Sides
[H] |
Sorting a list of shapes means checking each name against the rule. |
| Naming One Equal Part
[H] |
Two equal parts are halves and four equal parts are fourths. |
| How Many Pieces
[H] |
The name of the part tells how many pieces the whole was cut into. |
| Fourths and Quarters
[H] |
Fourth and quarter are two words for the same part of a whole. |
| Reading One Row of a Tally Chart
[H] |
Tally marks come in gates of five, so count by fives and then by ones. |
| Which Group Has the Most
[H] |
The biggest group is the row with the most tally marks. |
| Which Group Has the Fewest
[H] |
The smallest group is the row with the fewest pictures. |
| How Many More Chose It
[H] |
Match the two rows picture for picture; the extras are the difference. |
| How Many Altogether
[H] |
Adding every row of a chart gives how many children were counted. |
| The Group That Is Not Shown
[H] |
Take the shown groups away from the total to find the hidden group. |
| Sorting a Class List into Groups
[H] |
Sorting a list means counting the rows that belong to each group. |