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# Add Using Mental Math ## Key Concepts

• Break apart addends in different ways.
• Addition of numbers using mental math.

### Introduction

Break apart the addends to find the sum.

## Break apart numbers

### Addition of 2-digit number

Break apart the addends in different ways to find the sum.

Example 1:

Find 25 + 18.

Solution:

One way:

Break apart the second addend to make a 10.

Another way:

Break apart the second addend into tens and ones.

Add 25 + 10 = 35.

Next, 35 + 5 = 40.

Then, 40 + 3 = 43.

So, 25 + 18 = 43.

Example 2:

Find 37 + 14.

Solution:

One way:

Break apart the second addend to make a 10.

So, 37 + 14 = 51.

Another way:

Break apart the second addend into tens and ones.

Add 37 + 10 = 47.

Next, 47 + 3 = 50.

Then, 50 + 1 = 51.

So, 37 + 14 = 51.

Example 3:

Find 64 + 16.

Solution:

One way:

Break apart the second addend to make a 10.

So, 64 + 16 = 80.

Another way:

Break apart the second addend into tens and ones.

Add 64 + 10 = 74.

Then, 74 + 6 = 80.

So, 64 + 16 = 80.

### Model question

Question:

Toby has 34 crayons. Paul has 58 crayons. How many crayons do they have in all? Solve the problem mentally. Show the steps of breaking apart numbers to add.

Number of crayons with Toby = 34

Number of crayons with Paul = 58

The total crayons they have in all = 34 + 58

Find 34 + 58.

One way:

Break apart the second addend to make a 10.

34 + 58 = 92.

So, they have 92 crayons in all.

Another way:

Break apart the second addend into tens and ones.

Add 34 + 50 = 84.

Next, 84 + 6 = 90.

Then, 90 + 2 = 92.

34 + 58 = 92.

So, they have 92 crayons in all.

#### Activity

Break apart addends as tens and ones. Color the sheet as per the code.

Code:

80 + 9 = orange

60 + 7 = blue

40 + 8 = blue

70 + 5 = green

50 + 7 = green

30 + 6 = blue

## Exercise

Question 1:

Find the sum of 24 and 48 using mental math.

Question 2:

Find the sum of 39 and 17 using mental math.

Question 3:

Find using mental math. Draw pictures of blocks if needed.

Question 4:

Kevin plants 27 trees. Ronald plants 44 trees. How many trees do they plant in all?

Question 5:

Oliver has 23 marbles. Jack has 28 marbles. How many marbles do they have in all?

Question 6:

19 students are playing football. 23 students are skating. How many students are there in all?

Question 7:

Add tens and ones to solve. One number makes both the equations true. Find the missing number.

Question 8:

Add tens and ones to solve. One number makes both the equations true. Find the missing number.

31 + ⎕ = 44

⎕ + 44 = 57

Question 9:

Find 16 + 62.

Question 10:

Find 33 + 29.

### What we have learned

• Understand how to break apart addends in different ways to add.
• Add numbers using mental math by breaking apart the addends.

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