Adding and Subtracting Mentally Worksheet

Question 1

Find 27 + 10 in your head. Write the answer.

Accepted Answer

27 + 10 = 37. Adding 10 increases the tens digit by 1 and keeps the ones digit the same.

Question 2

Find 46 - 10 in your head. Write the answer.

Accepted Answer

46 - 10 = 36. Subtracting 10 decreases the tens digit by 1 and keeps the ones digit the same.

Question 3

Use a make-ten strategy to solve 8 + 7.

Accepted Answer

8 + 7 = 15. You can move 2 from 7 to 8 to make 10, then add the 5 left over to get 15.

Question 4

Solve 35 + 4 mentally.

Accepted Answer

35 + 4 = 39. Add the 4 ones to the 5 ones to get 9 ones, so the answer is 39.

Question 5

Solve 63 - 5 mentally.

Accepted Answer

63 - 5 = 58. You can subtract 3 to reach 60, then subtract 2 more to get 58.

Question 6

Find 24 + 6. Explain the quick way you used.

Accepted Answer

24 + 6 = 30. A quick way is to know that 24 needs 6 more to make the friendly number 30.

Question 7

Find 50 - 7 in your head.

Accepted Answer

50 - 7 = 43. You can subtract 5 to get 45, then subtract 2 more to get 43.

Question 8

Use doubles to solve 6 + 7.

Accepted Answer

6 + 7 = 13. Since 6 + 6 = 12, one more makes 13.

Question 9

Solve 42 + 18 mentally. Show how you can make a friendly number.

Accepted Answer

42 + 18 = 60. You can take 8 from 18 to make 42 into 50, then add the 10 left over to get 60.

Question 10

Solve 75 - 20 mentally.

Accepted Answer

75 - 20 = 55. Subtracting 2 tens from 7 tens leaves 5 tens, and the 5 ones stay the same.

Question 11

Mia has 32 stickers. She gets 9 more stickers. How many stickers does she have now? Use a mental math strategy.

Accepted Answer

Mia has 41 stickers now. You can add 8 to 32 to make 40, then add 1 more to get 41.

Question 12

A bus has 48 people on it. At the next stop, 6 people get off. How many people are on the bus now?

Accepted Answer

There are 42 people on the bus now. 48 - 6 = 42 because subtracting 6 ones from 8 ones leaves 2 ones.

Question 13

Find 29 + 14 mentally. Explain your strategy.

Accepted Answer

29 + 14 = 43. You can add 1 to 29 to make 30, then add the remaining 13 to get 43.

Question 14

Find 84 - 19 mentally. Explain your strategy.

Accepted Answer

84 - 19 = 65. You can subtract 20 from 84 to get 64, then add 1 back because you subtracted 1 too many.

Question 15

Choose the easier way to solve 57 + 23 mentally, then write the answer.

Accepted Answer

57 + 23 = 80. One easy way is to add 20 to 57 to get 77, then add 3 more to get 80.

Adding and Subtracting Mentally