Question Video: Estimating the Sum of Two Three-Digit Numbers by Rounding to the Nearest Hundred Mathematics • 3rd Grade

Look at how Noah rounded to estimate this sum. Round the numbers in this sum to the nearest hundred. Calculate the estimate.

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Video Transcript

Look at how Noah rounded to estimate this sum. And we’re shown an example. Round the numbers in this sum to the nearest hundred. And then we’re given a sum. Calculate the estimate.

Before we solve the problem, let’s look at how Noah estimates the calculation that he’s given by rounding to the nearest hundred. He starts off with the addition 432 plus 279. Instead of calculating the addition exactly, Noah, obviously, wants to find a rough answer. And that’s why he’s going to estimate.

If we were to put the number 432 on a number line, it would come between 400 and 500. 32 is less than 50, so 432 is nearer to 400 than 500. And so, Noah rounds his first number from 432 down to 400. Let’s have a look at the second number. On a number line, 279 comes somewhere between 200 and 300. 79 is greater than 50, so it’s past the halfway point. 279 is nearer to 300 than 200. And so, Noah rounds 279 up to 300.

By rounding his numbers to the nearest hundred, he’s changed the calculation from 432 plus 279 to 400 plus 300, which is a lot easier to work out. 400 plus 300 equals 700. The actual answer to Noah’s first calculation is 711. So, we can see that, by rounding to the nearest hundred, it gives a very close estimation.

Let’s now solve the problem. First, we’re asked to round the numbers in the sum to the nearest hundred. The first number, 527, comes between 500 and 600. 527 is less than 550, and so the nearest hundred is 500. 527 rounded to the nearest hundred is 500. Let’s round our second number. 164 comes between 100 and 200. 164 is more than 150, the halfway point. So, it’s nearest to 200. So, we can round the second number up to 200. And so, by rounding both numbers to the nearest hundred, the sum becomes 500 plus 200.

Finally, we’re asked to calculate the estimate. What is 500 plus 200? We know that five plus two is seven, so 500 plus 200 is 700. The actual answer to the calculation is 691, so we can see that this estimation is very close.

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