Rounding and Estimating
Choose and use estimation and rounding to check and explain the reasonableness of calculations including the results of financial transactions.
Read this first
Round to a tidy number first. Then check your real answer sits close to it.
67 sits past the middle, so it rounds up to 70.
Words you need
- Round
- Change a number to a tidy one nearby.
- Estimate
- A quick answer that is close, not exact.
- Reasonable
- An answer that makes sense.
Worked example
e.g. Round 67 to the nearest ten.
- Find the two tens either side. They are 60 and 70.
- The middle is 65. Is 67 before or after the middle?
- 67 is after 65. So it goes up.
- 67 rounds to 70.
If the number is exactly halfway, round up.
Practice
The pictures fade as you go. By the last few you are on your own.
1Round 43 to the nearest ten.with a picture
Answer40. It is before the middle.
2Round 85 to the nearest ten.with a picture
Answer90. Halfway rounds up.
3Round 128 to the nearest hundred.with a hint
Need a hint?
The two hundreds are 100 and 200.
Answer100.
4Estimate 39 + 42.with a hint
Need a hint?
Round each one to the nearest ten first.
Answer40 + 40 = 80.
5Estimate 21 x 5.on your own
Answer20 x 5 = 100.
6A calculator says 19 + 22 = 401. Is that sensible?on your own
AnswerNo. 20 + 20 is about 40, not 401.
7Estimate the cost of 3 items at $4.90 each.on your own
AnswerAbout 3 x 15.
Think about it
1
Why estimate before working something out?
One good answerSo you know straight away if your answer is silly.
2
Is an estimate wrong?
One good answerNo. It is meant to be close, not exact.
Common mistakes
Rounding down every time.
Look at the middle. Past the middle means round up.
Estimating after you finish.
Estimate first. Then the estimate can catch a mistake.