Mental math is math that you do without using a pencil and paper. Friendly numbers and skip counting can help you add and subtract using mental math.

**To better understand mental math within 100…**

## LET’S BREAK IT DOWN!

### What are friendly numbers?

Friendly numbers are numbers that are easy to add. It is easy to add 10 and another number. 10 + 2 = 12. It is easy to add together numbers that make a ten. 4 + 6 = 10. Doubles are also easier to add. 6 + 6 = 12.

### Find how many crayons in all.

You have 11 crayons. A friend has 7 crayons. How many crayons in all? 11 + 7 = ? To make friendlier numbers, take 1 away from 11 to get 10. Then move 1 to the 7 to get 8. 10 + 8 = 18. There are 18 markers in all.

### Find how many flowers in all.

You have 8 red flowers and 9 blue flowers. How many flowers in all? Use a double fact: 8 + 8 = 16. 8 + 9 is one more, so 8 + 9 = 17.

### Find how many mice are left.

There are 24 mice in a basket. 11 mice leave. How many are left? Find 24 – 11. It is easy to subtract 10. 11 is 10 and 1 more. 24 – 10 = 14. 14 – 1 = 13. There are 13 mice left.

### Towers

There is a tower of 20 blocks. You add stacks of 10 blocks on top. To add groups of 10 you can skip count by 10. 20 + 10 = 30. To add 34 + 50, skip count by 10. Start at 34, then jump to 44, 54, 64, 74, 84. So, 34 + 50 = 84.

## MENTAL MATH WITHIN 100 VOCABULARY

## MENTAL MATH WITHIN 100 DISCUSSION QUESTIONS

### How can we use mental math to add 6 + 9?

### Is there an easier way to subtract 20 – 17 than counting backward from 20?

### How can we use mental math to subtract 11 – 4?

### What happens to the tens place when we add 40 + 30? What happens to the ones place?

### What happens to the tens place when we subtract 53 – 20? What happens to the ones place?

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