Myriora Maths › Maths foundations › Chapter 2: Adding

Is my answer sensible?

Ages 7–9Chapter 2 · Lesson 7

In this lesson

Round the numbers, and add them in your head.
The real answer should be close to your estimate.
And the ones digits give you another quick check.

Your turn

Which answer is sensible for two hundred and ninety-one, plus five hundred and two?

Show the answer

Two hundred and ninety-one is about three hundred. Five hundred and two is about five hundred. So the answer is about eight hundred. Seven hundred and ninety-three is the sensible one!

The common mistake

Here's a question that catches lots of people. Estimate three hundred and ninety-eight, plus two hundred and five. Some people just drop the tens and ones, and say three hundred plus two hundred: five hundred. But three hundred and ninety-eight is almost four hundred! Round to the nearest hundred: four hundred plus two hundred is six hundred.

Try it at home

Before using a calculator, guess the answer by rounding. Was the calculator answer close?

Read the whole lesson

Hello, and welcome to Myriora Maths! Someone says three hundred and ninety-eight, plus two hundred and five, is eight hundred and three. Is that sensible? You can tell without working it all out!

First, round each number to make it easy. Three hundred and ninety-eight is about four hundred. Two hundred and five is about two hundred. Four hundred plus two hundred is six hundred. So the answer should be about six hundred.

Eight hundred and three is a long way from six hundred. So something has gone wrong! The real answer is six hundred and three, which is very close to our estimate.

Why does this work? Rounding changes each number only a little. So the estimate lands close to the real answer. If an answer is far from the estimate, check it again.

Let's try one together. Is fifty-one plus forty-eight about one hundred, or about two hundred? Round both numbers first. Say it out loud!

Fifty-one is about fifty. Forty-eight is about fifty too. Fifty plus fifty is one hundred. So the answer is about one hundred! It's actually ninety-nine.

Here's another quick check: the ones digits. Three hundred and ninety-eight ends in eight, and two hundred and five ends in five. Eight plus five is thirteen, so the answer must end in three. Six hundred and three ends in three. Good!

Your turn! Which answer is sensible for two hundred and ninety-one, plus five hundred and two? Pick the right card. You've got fifteen seconds, or pause the video. Go!

Time's up! Two hundred and ninety-one is about three hundred. Five hundred and two is about five hundred. So the answer is about eight hundred. Seven hundred and ninety-three is the sensible one!

Quick review. Round the numbers, and add them in your head. The real answer should be close to your estimate. And the ones digits give you another quick check. See you next time!