Mental Math Within 100 — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. 25 + 40 = 65 using the pattern of adding four tens.
Four tens added to 25 gives 65, following the add-four-tens pattern.
2. 35 + 40 = 75 by jumping four tens from 35.
Four tens added to 35 lands on 75, keeping the same ones digit but raising the tens by 4.
3. 45 + 40 = 85 continuing the same pattern.
45 has 4 tens, and adding 4 more tens makes 8 tens plus 5 ones, which is 85.
4. The ones digit stays the same in these patterns. It is always 5.
Each starting number ends in 5, and adding whole tens does not change the ones digit, so the answer ends in 5.
5. 16 + 20 = 36 by adding two tens.
16 plus 2 tens is 36, keeping the ones digit 6 and raising tens by 2.
6. 26 + 20 = 46 by the same pattern as 16 + 20.
26 with 2 more tens gives 46, following the same add-two-tens pattern.
7. 36 + 20 = 56 continuing the pattern.
36 plus 2 tens is 56, the next number in the pattern ending in 6.
8. 50 + 30 = 80 follows a pattern of pure tens.
5 tens plus 3 tens is 8 tens, so the sum is 80.
9. 60 + 30 = 90 continues the pure tens pattern.
6 tens plus 3 tens is 9 tens, which equals 90 in the pattern.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
22 + 30
→ 52
52
32 + 30
→ 62
62
42 + 30
→ 72
72
52 + 30
→ 82
82
Adding 30 to each starting number raises the tens by 3 while keeping the ones digit 2, giving 52, 62, 72, and 82.