Skill: Use estimation to check reasonableness

Estimation helps you predict about how much an answer should be. Then you can compare the exact answer with your estimate to see if it makes sense.

Example: A library has $(623)$ books. It lends $(278)$ books. About how many books are left?

1. Choose the operation.
   Books are being taken away, so subtract:

$$623-278$$

2. Round to the nearest hundred.

$$623\approx 600 \qquad 278\approx 300$$

3. Find the estimate.

$$600-300=300$$

So, about $(300)$ books should be left.

4. Find the exact answer.

$$623-278=345$$

5. Check the answer.
   The exact answer, $(345)$, is reasonably close to the estimate, $(300)$. It makes sense that about $(300)$ books are left.

An estimate does not have to equal the exact answer. It gives you a quick number to use for checking whether your answer is reasonable.