Adding and Subtracting Mentally
Use friendly numbers, doubles, and place value to solve in your head
Adding and Subtracting Mentally
Use friendly numbers, doubles, and place value to solve in your head
Math - Grade 2-3
- 1
Find 27 + 10 in your head. Write the answer.
Count up one ten from 27.
27 + 10 = 37. Adding 10 increases the tens digit by 1 and keeps the ones digit the same. - 2
Find 46 - 10 in your head. Write the answer.
46 - 10 = 36. Subtracting 10 decreases the tens digit by 1 and keeps the ones digit the same. - 3
Use a make-ten strategy to solve 8 + 7.
Make 8 into 10 first.
8 + 7 = 15. You can move 2 from 7 to 8 to make 10, then add the 5 left over to get 15. - 4
Solve 35 + 4 mentally.
35 + 4 = 39. Add the 4 ones to the 5 ones to get 9 ones, so the answer is 39. - 5
Solve 63 - 5 mentally.
Break 5 into 3 and 2.
63 - 5 = 58. You can subtract 3 to reach 60, then subtract 2 more to get 58. - 6
Find 24 + 6. Explain the quick way you used.
Think about how many more are needed to reach 30.
24 + 6 = 30. A quick way is to know that 24 needs 6 more to make the friendly number 30. - 7
Find 50 - 7 in your head.
50 - 7 = 43. You can subtract 5 to get 45, then subtract 2 more to get 43. - 8
Use doubles to solve 6 + 7.
Start with the double 6 + 6.
6 + 7 = 13. Since 6 + 6 = 12, one more makes 13. - 9
Solve 42 + 18 mentally. Show how you can make a friendly number.
Make 42 into 50 first.
42 + 18 = 60. You can take 8 from 18 to make 42 into 50, then add the 10 left over to get 60. - 10
Solve 75 - 20 mentally.
75 - 20 = 55. Subtracting 2 tens from 7 tens leaves 5 tens, and the 5 ones stay the same. - 11
Mia has 32 stickers. She gets 9 more stickers. How many stickers does she have now? Use a mental math strategy.
Try making 40 first.
Mia has 41 stickers now. You can add 8 to 32 to make 40, then add 1 more to get 41. - 12
A bus has 48 people on it. At the next stop, 6 people get off. How many people are on the bus now?
There are 42 people on the bus now. 48 - 6 = 42 because subtracting 6 ones from 8 ones leaves 2 ones. - 13
Find 29 + 14 mentally. Explain your strategy.
Make 29 into 30 first.
29 + 14 = 43. You can add 1 to 29 to make 30, then add the remaining 13 to get 43. - 14
Find 84 - 19 mentally. Explain your strategy.
Subtract 20, then fix the answer.
84 - 19 = 65. You can subtract 20 from 84 to get 64, then add 1 back because you subtracted 1 too many. - 15
Choose the easier way to solve 57 + 23 mentally, then write the answer.
57 + 23 = 80. One easy way is to add 20 to 57 to get 77, then add 3 more to get 80.