Mental Math Within 100 — Answer Key
Part A: Sort the Words
Sort each word or number into the correct category box.
1. Sort each problem by whether you count on by tens or by ones.
Count on by tens
56 + 2030 + 2740 + 25 Count on by ones
43 + 371 + 462 + 2 Adding 20 means two jumps of ten, while adding 3 means three little jumps of one.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. Start at 56 and count on 20 by tens: 56, 66, 76.
After 66 one more ten gives 76, so 56 + 20 equals 76.
2. Start at 34 and count on by tens three times: 34, 44, 54, 64.
Three jumps of ten from 34 lands on 64, so 34 + 30 is 64.
3. 47 + 30 by counting on tens equals 77.
47, 57, 67, 77 shows three tens added, giving a sum of 77.
4. Count on by ones: 82 + 3 = 85.
82, 83, 84, 85 is three ones added, so the sum is 85.
5. 20 + 63 counted as tens then ones is 83.
63 plus two tens is 83, so 20 + 63 equals 83.
Part C: True or False?
Read each statement. Circle True or False.
1. Counting on by tens means the ones digit stays the same.
True False
Jumping by ten only changes the tens place, so the ones digit stays.
2. 56 + 20 gives the same answer as 56, 66, 76.
True False
Two tens added to 56 is 76, matching the count-on steps.
3. To add 45 + 4 quickly, you should count by tens.
True False
4 is only four ones, so count on by ones, not by tens.
4. Counting on is slower than using fingers for every number.
True False
Counting on skips over many small steps, so it is faster than one-by-one.