Mental Math Within 100 — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. 35 + 8: first add 5 to reach 40, then 3 more. Sum is 43.
35 + 5 is 40 and 40 + 3 is 43, so 35 + 8 equals 43 by splitting 8 into 5 and 3.
2. 28 + 7 = 28 + 2 + 5, which equals 35.
28 + 2 is 30 and 30 + 5 is 35, so splitting 7 into 2 plus 5 helps reach the answer.
3. 47 + 6 = 47 + 3 + 3 = 53.
47 + 3 is 50 and 50 + 3 is 53, giving 47 + 6 = 53 using the make-a-ten idea.
4. To solve 69 + 5, first add 1 to reach 70.
69 needs 1 more to reach 70, so split 5 into 1 plus 4 to build back to ten.
5. 69 + 5 = 69 + 1 + 4 = 74.
69 + 1 is 70 and 70 + 4 is 74, so 69 + 5 equals 74 through building back to ten.
6. 56 + 9 = 56 + 4 + 5, which gives 65.
56 + 4 is 60 and 60 + 5 is 65, so splitting 9 into 4 plus 5 reaches the sum.
7. 72 + 8 uses 8 = 8 + 0 because 72 + 8 already lands on a ten.
2 + 8 makes 10, so 72 + 8 builds straight to 80 without needing to split the 8.
8. 48 + 7 = 48 + 2 + 5 to finish the make-a-ten step.
After adding 2 to reach 50, there are 5 left from the original 7, since 2 plus 5 is 7.
9. 29 + 6 = 29 + 1 + 5 = 35.
29 + 1 is 30 and 30 + 5 is 35, so 29 + 6 equals 35 using build-back-to-ten.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
38 + 5
→ 38 + 2 + 3
38 + 2 + 3
46 + 7
→ 46 + 4 + 3
46 + 4 + 3
59 + 4
→ 59 + 1 + 3
59 + 1 + 3
27 + 6
→ 27 + 3 + 3
27 + 3 + 3
Each split first fills to the next ten (40, 50, 60, 30) and then adds the leftover ones, which gives the correct strategy.