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.  

Answer:  

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.

Concept Map

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.