Mental Math Within 100 — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. 48 + 19 can become 48 + 20, then subtract 1.
19 was rounded up to 20, which is 1 too many, so you subtract 1.
2. 48 + 20 = 68, then 68 - 1 = 67.
Since 20 was one more than 19, remove that extra to land on 67.
3. 35 + 29 becomes 35 + 30 - 1 = 64.
35 + 30 is 65, and subtracting the extra 1 gives 64.
4. 44 + 39 becomes 44 + 40 - 1 = 83.
44 + 40 is 84, and 1 less is 83 since you added one too many.
5. 27 + 18 becomes 27 + 20 - 2 = 45.
27 + 20 is 47, then subtract 2 extras to reach 45.
6. If you change 49 + 32 into 50 + 32 - 1, fill in the blank.
49 rose by 1 to become 50, so remove that 1 at the end.
7. Using compensation, 25 + 48 = 25 + 50 - 2 = 73.
25 + 50 is 75 and taking away 2 extras gives 73, which equals 25 + 48.
8. To add 63 + 19 use 63 + 20 - 1 = 82.
63 + 20 is 83 and subtracting 1 extra gives 82.
9. Compensation means you change one number to make it easier to add.
You trade a messy number for a friendly ten so the sum is easier to find in your head.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
46 + 19
→ 46 + 20 - 1
46 + 20 - 1
27 + 29
→ 27 + 30 - 1
27 + 30 - 1
52 + 38
→ 52 + 40 - 2
52 + 40 - 2
35 + 48
→ 35 + 50 - 2
35 + 50 - 2
Each problem rounds the second addend up and then subtracts the amount that was added extra.