Mental Math Within 100 — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. Use parts to solve 65 - 22. First do 65 - 20, then subtract 2. What is the answer?
A) 43
B) 45
C) 47
D) 53
Subtracting 20 from 65 gives 45, and subtracting 2 more gives 43.
2. Which is the quickest mental strategy for 80 - 39?
A) Count on by ones
B) Subtract 40, then add 1 back
C) Double 39
D) Add 80 + 39
80 - 40 is 40, and adding 1 back gives 41, which is 80 - 39 using compensation.
3. Count back from 78 by tens three times. What number do you land on?
A) 58
B) 48
C) 38
D) 28
78, 68, 58, 48 — three tens back from 78 lands on 48.
4. Which split is best for mentally finding 94 - 37?
A) 94 - 30 - 7
B) 94 - 7 - 30
C) 94 + 30 - 7
D) 94 + 7 + 30
Break 37 into 30 and 7, then subtract in order: 94 - 30 is 64 and 64 - 7 is 57.
Part B: Fill in the Blank
Write the correct answer on each line.
1. 70 - 25 by parts is (70 - 20) - 5 = 45.
70 - 20 is 50 and subtracting 5 more gives 45, matching the full problem.
2. Count back by tens: 83, 73, 63, 53. So 83 - 30 = 53.
Three tens removed from 83 leaves 53, which is 83 - 30.
3. Use compensation: 62 - 19 = 62 - 20 + 1 = 43.
62 - 20 is 42, and adding 1 back gives 43 since 19 is 1 less than 20.
4. 90 - 48 using compensation is 90 - 50 + 2 = 42.
90 - 50 is 40, and adding 2 back gives 42 because 48 is 2 less than 50.
5. 56 - 24 split into parts is 56 - 20 - 4 = 32.
56 - 20 is 36 and 36 - 4 is 32, which equals 56 - 24.