We can estimate the answer to an addition sum by rounding the numbers to the nearest ten before we add.

 

Remember that a number that ends in five or above rounds up to the next ten.

A number that ends in four or below rounds down to the previous ten.

 

Example 1

Estimate the answer to the following sum by rounding the numbers to the nearest ten and then adding.

 

26 + 47

 

Answer

26 + 47

 ↓      ↓

30 + 50 = 80

 

Example 2

Estimate the answer to the following sum by rounding the numbers to the nearest ten and then adding.

 

94 + 75

 

Answer

94 + 75

 ↓      ↓

90 + 80 = 170

 

Remember that 75 rounds up to 80, 35 rounds up to 40, 25 rounds up to 30 etc. 

 

Let's have a go at some questions now.