Mental Math Within 100 — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. Double 25 means 25 + 25 = 50.
25 plus another 25 equals 50, which is the double of 25.
2. Double 30 is 60.
Two groups of 30 make 60, so double 30 is 60.
3. A near-double like 25 + 26 equals (25 + 25) + 1 = 51.
The double 50 plus one more ten is just 51 since 26 is one above 25.
4. 24 + 25 uses the double of 24, which is 48, plus 1. The sum is 49.
Double 24 is 48, and 25 is one more than 24, so 48 + 1 equals 49.
5. Double 15 is 30.
10 + 10 is 20 and 5 + 5 is 10, so 15 + 15 is 30.
6. 40 + 41 = double 40 + 1 = 81.
Double 40 is 80, and 41 has one more than 40, making the sum 81.
7. Double 35 = 35 + 35 = 70.
30 + 30 is 60 and 5 + 5 is 10, which together make 70.
8. Near-double 19 + 20 is (double 19) + 1 = 38 + 1 = 39.
Double 19 is 38, and one more makes 39 to match adding 20.
9. Double 45 is 90.
40 + 40 is 80 and 5 + 5 is 10, so double 45 equals 90.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Double 20
→ 40
40
Double 22
→ 44
44
23 + 24
→ 47
47
30 + 31
→ 61
61
Double 20 is 40, double 22 is 44, 23 + 24 is double 23 plus 1 which is 47, and 30 + 31 is 60 + 1 equals 61.