Use Rounding to Check if Answers to Questions are Reasonable

In this activity, we'll be rounding numbers (including decimals) to estimate and check the answers to questions.

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

If our estimate and answer are very different, it is likely 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 100, we look at the tens digit.

If we are rounding to the nearest 10, we look at the ones 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 3.5 rounded to the nearest whole number?

The digit to the right of the decimal point is in the tenths column and is a 5.

So 3.5 would be rounded up to 4.

Note: The number rounds to a whole number.

What is 56 rounded to the nearest 10?

The digit to the right of the tens digit is in the ones column and is a 6.

So 56 would be rounded up to 60.

Note: The number rounds up to the next multiple of 10 (up to 60).

What is 734 rounded to the nearest 100?

The digit to the right of the hundreds digit is in the tens column and is a 3.

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 couple of  questions using rounding.

The answer to 32 x 47 = 1,504

Round both numbers to the nearest 10 to check if the answer is reasonable.

32 rounds down to 30

47 rounds up to 50

30 x 50

To do this we can do the following:

3 x 5 = 15

30 x 5 = 150

30 x 50 = 1,500

We can see that the actual answer (1,504) is very close to the estimate, so we can assume it is probably correct!

Now let's look at a word problem.

Charlie raised £654.20 for charity and Clara raised a further £213.50

Estimate the answer by rounding each amount to the nearest £10 before adding.

£654.20 rounds down to £650

£213.50 rounds down to £210 

£650 +  £210 = £860

The actual answer is £867.70

Can you see how estimating answers helps us to see if our answers are reasonable?

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