Mental Math Within 100 — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. What is the best mental step for 52 - 28?
A) 52 - 30 + 2
B) 52 - 30 - 2
C) 52 + 30 - 2
D) 52 - 20 - 8 by ones
52 - 30 is 22, and adding 2 back gives 24 since 28 is 2 less than 30.
2. To solve 63 - 27 by counting up, what is the first friendly jump?
A) From 27 to 30
B) From 27 to 50
C) From 27 to 27
D) From 27 to 63
Counting up 3 to 30, then 30 more to 60, then 3 more to 63 gives 36 as the difference.
3. Which strategy works best for 81 - 49?
A) 81 - 50 + 1
B) 81 - 50 - 1
C) 81 + 50 - 1
D) 81 - 40 - 9 by ones
81 - 50 is 31, and adding 1 back gives 32 because 49 is 1 less than 50.
4. Which is the best break-apart for 74 - 36?
A) 74 - 30 - 6
B) 74 - 6 - 30
C) 74 + 30 - 6
D) 74 - 36 on fingers
74 - 30 is 44, and 44 - 6 is 38, which equals 74 - 36 by breaking the subtrahend apart.
Part B: Fill in the Blank
Write the correct answer on each line.
1. A box has 52 crayons. 28 are used. Remaining crayons: 52 - 30 + 2 = 24.
52 - 30 is 22, and adding 2 back gives 24 because 28 is 2 less than 30.
2. A bag has 63 beads and 27 fall out. Count up 3 to 30, then 30 to 60, then 3 to 63. Beads lost: 36.
The jumps total 36, which is also 63 minus 27, so the lost count is 36 beads.
3. There are 81 chairs and 49 are in use. Empty chairs: 81 - 50 + 1 = 32.
81 - 50 is 31, and adding 1 back gives 32, which equals 81 - 49 by compensation.
4. A shelf has 74 books. 36 are loaned out. Remaining: 74 - 30 - 6 = 38.
74 - 30 is 44 and 44 - 6 is 38, so 38 books remain on the shelf.
5. A pond had 90 frogs and 47 hopped away. Remaining: 90 - 50 + 3 = 43.
90 - 50 is 40, and adding 3 back gives 43 because 47 is 3 less than 50.