Use Rounding to Estimate Answers

In this activity, we'll be rounding whole numbers to estimate the answers to addition questions.

 

Estimating can help us to decide if an answer to a problem is reasonable. 

If our estimate and answer are very different, it is likely that we have made an error.

 

Recap of rounding

When rounding, we look at the digit to the right of the number we are rounding to.

If we are rounding to the nearest 10, we look at the ones digit, if rounding to the nearest 100, we look at the tens digit.

We round up or down depending on the value of this digit.

 

If this digit is less than 5, the number needs to be rounded down.

If this digit is 5 or above, the number needs to be rounded up.

 

 

Let's have a look at some examples:

 

What is 56 rounded to the nearest 10?

When rounding to the nearest 10, we need to look at the digit in the ones column.

In 56, this is a 6, so we round up.

So 56 would be rounded up to 60

 

What is 734 rounded to the nearest 100?

When rounding to the nearest 100, we look at the digit in the tens column.

There is a 3 in the tens column, so we round down.

734 would be rounded down to 700 because the tens digit is less than 5.

Note: The number rounds down to 700, not the previous 100 which would be 600!

 

 

Let's have a look at a question using rounding.

 

Estimate the answer to the addition by first rounding the numbers to the nearest ten.

 

27 + 32 =

 

We're rounding to the nearest 10, so we look at the digit in the ones column.

27 has a 7 in the ones column, so we round up to 30.

32 has a 2 in the ones column, so we round down to 30.

30 + 30 = 60

 

27 + 32 =   30 + 30 = 60

 

The actual answer is:  27 + 32 = 59

 

We can see that our answer is reasonable and probably accurate.

 

Now it's your turn to have a go at some questions using rounding to estimate answers.