Estimating Numbers

Lesson Plan - Get It!

Audio:

When you estimate, you find a number that is close to the correct answer.

An estimation is not just a guess — it has to be reasonable.

• Would it be reasonable to estimate that there are 15 jelly beans in the jar above?

No. You can tell that there are more than 15 jelly beans. So your estimate needs to be closer to the correct answer.

Estimation is a lot like rounding numbers. If you want to estimate to the nearest tens place, then you would look to the digit in the ones place to see if you should round up or down. So 329 would become 330.

The same is true if you wanted to estimate to the nearest hundreds place. You would look at the digit in the tens place to see if you should round the number in the hundreds place up or down. Here, 231 would become 200.

• When do you think estimates would be useful in math?

Estimates are very helpful when you want to solve math problems. Instead of adding 239 and 344, you can estimate. Finding the sum of 240 and 340 is a much easier problem to solve.

If you estimate, you see that 240 + 340 = 580. The actual sum of 239 + 344 = 583, so your estimation was close to the actual answer.

You can estimate to solve addition, subtraction, multiplication, and division problems.

Take a look at an example.

Kara made 420 cookies for her class. She sold 249 cookies. About how many cookies are left?

When you see the words “about how many” or “choose the best estimate,” you know to estimate.

Instead of subtracting 420 - 249, you can simply estimate to the nearest hundred: 400 - 200 = 200.

Kara had about 200 cookies left.

Watch the Cyberchase video below to see how Inez uses estimation to decide if she has enough jelly beans to bake cookies for her class.

• How did Inez use estimation in the video?
• Was her estimate reasonable?

Move on to the Got It? section to try out estimation on your own.