Idea 18of 24Builds onThe algorithm in two rows
Estimate to check a product
How can you tell a product is wrong without redoing it?
Today you'll discover
- how to round each factor
- how big the product should be
- which slips an estimate catches
Think first47 × 18, with an estimate
47 × 18 is close to 50 × 20 = 1000. Which answer is reasonable?
Decide first, then press Next. Estimate first, then compare.
47 × 18 written in a column, with the estimate 50 × 20 = 1000 at the lower right. The four partial products 56, 320, 70 and 400 appear, then the same in two rows, and both columns total 846, close to 1000. The last step shows 62 × 19.
See the whole ideathe big idea and the key word, in one place
The big idea
Round both factors to the nearest ten and multiply: the product should be near that.
A class trip47 students pay $18 each. About 50 × $20 = $1000, so a total of $846 makes sense.
Easy to get wrongIf the answer is about ten times the estimate, or a tenth of it, look for a zero that is missing or extra.
Key word
- reasonable
- about the right size: close to what an estimate says it should be
Ready to use it?
- Estimate a two-digit product by rounding.
- Decide whether a product is reasonable.