Round Numbers to Estimate Addition and Subtraction Problems

It can be useful to estimate the answer to a question.

We often do this to see if the actual answer is likely to be correct - the estimation should be quite close to the answer.

We can use rounding to help us make a good estimation of an answer.

Some numbers need to be rounded up and others need to be rounded down.

Rounding to the nearest ten:

If a two-digit number ends in 5 or above, it rounds up to the next ten (35 rounds up to 40).

If a two-digit number ends in 4 or below it rounds down to the previous ten (32 - rounds down to 30).

Rounding to the nearest hundred:

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

If it is 5 or above, we round up (367 - rounds up to 400).

If it is 4 or below, we round down (345 - rounds down to 300).

If we are rounding to the nearest 1,000, we would look at the digit in the hundreds column.

If we are rounding to the nearest 10,000, we would look at the digit in the thousands column.

Can you see the pattern?  We look at the column to the right of the digit we are rounding to!

Example 1

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

453 + 326 

453 rounds up to 500

326 rounds down to 300

Now add the numbers 500 + 300 = 800

The actual answer is 453 + 326 = 779 (a very close estimate).

Example 2

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

65 - 41

65 rounds up to 70

41 rounds down to 40

Now subtract 70 - 40 = 30

The actual answer is 65 - 41 = 24 (a good estimate).

Are you ready to try some questions now?