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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.
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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 by rounding the numbers to the nearest hundred and then subtracting.
683 - 342
683 rounds up to 700
322 rounds down to 300
Now subtract the numbers 700 - 300 = 400
The actual answer is 683 - 322 = 361 (quite a good estimate).
We could try rounding to the nearest ten to see if it is closer.
683 rounds down to 680
322 rounds down to 320
680 - 320 = 360 (we can see that this is a better 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).
The closer the starting number is to the number we are rounding to, the better the estimate will be.
Are you ready to try some questions now?
