Idea 22of 23Builds onThe little 1 is a ten moved upSubtraction undoes addition
Estimate to check an answer
How can you tell an answer is wrong without working it out?
Today you'll discover
- how to round to the nearest thousand
- how close an answer should be
- two ways to check
Think firstSam's answer to 38 476 + 25 859
Sam says 38 476 + 25 859 = 54 335. Is that reasonable?
Decide first, then press Next. Round each number before you judge.
38 476 above 25 859 in columns, with Sam's answer, 54 335, in orange under the line. An orange estimate line reads 38 000 + 26 000 = 64 000. Then the real working gives 64 335, near the estimate, and the check 64 335 − 25 859 = 38 476. The last step shows Lee's answer to 61 204 − 29 876.
See the whole ideathe big idea and the key word, in one place
The big idea
Round to friendly numbers first: a reasonable answer is close to the estimate.
Easy to get wrongAn estimate catches big slips, like a lost ten thousand. To catch a small slip, check with the inverse operation.
Key words
- estimate
- a close answer worked out with friendly numbers
- reasonable
- close to what an estimate says it should be
Ready to use it?
- Estimate to decide whether an answer is reasonable.
- Check an answer with the inverse operation.