Mental Math Within 100 — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. Which strategy works best for 34 + 29?
A) 34 + 30 - 1
B) 34 + 30 + 1
C) 34 - 30 + 1
D) 34 + 20 + 9 + 1
29 rounded up to 30 makes the sum 64, and subtracting the extra 1 gives the correct answer 63.
2. Which is the best break-apart step for 46 + 27?
A) 40 + 20 and 6 + 7
B) 46 + 20 + 7
C) 40 + 6 only
D) 46 - 27 and add back
Breaking into 40 + 20 and 6 + 7 gives 60 and 13, which combine to 73 as the mental sum.
3. Which approach is easiest for 58 + 19?
A) 58 + 20 - 1
B) 58 + 10 + 9
C) 58 + 19 on paper
D) 58 - 20 + 1
58 + 20 is 78, and subtracting the extra 1 gives 77, which equals 58 + 19 by compensation.
4. Which strategy is best for 25 + 38?
A) 25 + 40 - 2
B) 25 + 38 by ones
C) 25 + 30 + 10
D) 25 - 38 + 2
25 + 40 is 65, and subtracting 2 gives 63, which is the correct sum of 25 + 38.
Part B: Fill in the Blank
Write the correct answer on each line.
1. 34 + 29 using compensation is 34 + 30 - 1 = 63.
34 + 30 is 64, and subtracting the 1 extra from rounding gives 63.
2. 46 + 27 by break-apart: 40 + 20 = 60 and 6 + 7 = 13, sum is 73.
60 plus 13 is 73, so 46 + 27 equals 73 by splitting into tens and ones parts.
3. 58 + 19 by compensation is 58 + 20 - 1 = 77.
58 + 20 is 78, and subtracting the extra 1 gives 77, which equals 58 + 19.
4. 25 + 38 using compensation is 25 + 40 - 2 = 63.
25 + 40 is 65, and removing the extra 2 gives 63, matching 25 + 38.
5. 62 + 25 by break-apart: 60 + 20 = 80 and 2 + 5 = 7. Sum is 87.
80 plus 7 is 87, so 62 + 25 equals 87 using the break-apart mental strategy.