Money
Find totals and change.
Algebra Readiness · Unit 1
Mental Addition and Subtraction Within 20
Mental math means solving a problem in your head by changing it into numbers that are easier to use. There is more than one good strategy.
Why should I care?
Mental Math Helps You Think Faster
These strategies help you count money, keep score, estimate amounts, and check whether an answer makes sense. They also build the number sense needed for Algebra.
Scores
Track points during a game.
Shopping
Estimate how much you will spend.
Check Answers
Notice when an answer is unreasonable.
1
Strategy 1 · AR-U1-WS1
Count On
Start with the larger number and count forward.
8+19+110+111
8 + 3 = 11
- Start at 8.
- Count three more: 9, 10, 11.
- You land on 11.
Core Practice AR-U1-WS1 · 1–10 PDF ↓Complete Worksheet AR-U1-WS1 · 1–16 PDF ↓Teacher Answer Key AR-U1-WS1 · Sign in →
Interactive game
0 of 10 masteredFormula 1 Number-Line Race
Start at 4. Move 3 spaces forward, one number at a time.
0
1
2
3
🏎️4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Moves: 0 / 3
2
Strategy 2 · AR-U1-WS2
Doubles and Near Doubles
Use a doubles fact you know. For a near double, adjust by one.
6 + 7 = 13
- Remember: 6 + 6 = 12.
- The second number is one greater.
- Add one more: 12 + 1 = 13.
Core Practice AR-U1-WS2 · 1–10 PDF ↓Complete Worksheet AR-U1-WS2 · 1–16 PDF ↓Teacher Answer Key AR-U1-WS2 · Sign in →
Interactive game
0 of 10 masteredDoubles Card Match
Study the two cards. Choose the correct total.
+= ?
3
Strategy 3 · AR-U1-WS3
Make a Ten
Break apart one number so the other number can become 10.
+ 3 more
8 + 5 = 13
- Break 5 into 2 and 3.
- Use the 2 to complete 10: 8 + 2 = 10.
- Add what remains: 10 + 3 = 13.
Core Practice AR-U1-WS3 · 1–10 PDF ↓Complete Worksheet AR-U1-WS3 · 1–16 PDF ↓Teacher Answer Key AR-U1-WS3 · Sign in →
Interactive game
0 of 10 masteredDigital Ten-Frame Builder
Build 8 + 2. Fill the frame, count all counters, and enter the total.
Loose counters
4
Strategy 4 · AR-U1-WS4
Use Place Value
Keep the ten and solve with the ones.
10one ten
+3 + 5eight ones
= 1813 + 5 = 18
- 13 is 10 and 3.
- Add the ones: 3 + 5 = 8.
- Put the ten and ones together: 10 + 8 = 18.
Core Practice AR-U1-WS4 · 1–10 PDF ↓Complete Worksheet AR-U1-WS4 · 1–16 PDF ↓Teacher Answer Key AR-U1-WS4 · Sign in →
Interactive game
0 of 10 masteredMoney Builder
You have $11. Receive $6. Add or remove $10, $5, and $1 bills to show the new amount.
$10TEN DOLLARS
$1
Total: $115
Strategy 5 · AR-U1-WS5
Use Compensation
Change a number to make the problem easier, then correct the change.
9 + 6make 9 into 1010 + 6 = 16remove the extra 115
9 + 6 = 15
- Change 9 to the friendly number 10.
- Solve 10 + 6 = 16.
- You added one extra, so subtract it: 16 − 1 = 15.
Core Practice AR-U1-WS5 · 1–10 PDF ↓Complete Worksheet AR-U1-WS5 · 1–16 PDF ↓Teacher Answer Key AR-U1-WS5 · Sign in →
Interactive game
0 of 10 masteredFriendly-Number Transfer
Move counters from the second group to the first group until the first number becomes 10.
9
+2
How many counters should move?
6
Strategy 6 · AR-U1-WS6
Add Up in Chunks
Break a number into smaller parts that create an easy stopping point.
8break apart
35
7 + 3 + 5 = 157 + 8 = 15
- Break 8 into 3 and 5.
- Add the first chunk: 7 + 3 = 10.
- Add the second chunk: 10 + 5 = 15.
Core Practice AR-U1-WS6 · 1–10 PDF ↓Complete Worksheet AR-U1-WS6 · 1–16 PDF ↓Teacher Answer Key AR-U1-WS6 · Sign in →
Interactive game
0 of 10 masteredBreak-Apart Machine
Break 8 into two chunks. Choose the split that helps 7 reach 10 first.
7+
8
Choose a split ↓Strategy challenge
Choose a Strategy and Solve
You may use any strategy that makes sense to you. Type each answer, then check your work.
Remember
The best strategy is the one that helps you understand the numbers and solve the problem accurately. Different students may choose different strategies.