Video: Estimating Sums of Three-Digit Numbers

In this lesson, we will learn how to estimate the sums of three-digit numbers.

07:04

Video Transcript

Estimating Sums of Three-Digit Numbers

In this video, we will learn how to add together two three-digit numbers using rounding and estimating. Let’s look at an example. Look at how Amelia estimated this sum: estimate 273 plus 146. First, round 273, rounded to the nearest 10 is 270. 146 rounded to the nearest 10 is 150. Then calculate 270 plus 150, which equals 420. So we’re learning how to add using estimation. First, we have to round both of the three-digit numbers to the nearest 10.

Let’s have a closer look at how Amelia did that. First Amelia rounded 273 to 270. We can draw a number line to help us round. 273 is between 270 and 280 on our number line. The number line helps us to see that 273 is nearest to 270 on the number line. When we’re rounding to the nearest 10, we always look at the ones digit. If the ones digit is less than five, then we round down. 273 has a three in the ones digit, which is less than five, so we rounded down to 270.

Next Amelia rounded 146 to the nearest 10, which was 150. 146 has six ones. As six is greater than five, we have to round up to the next 10. So when we’re rounding, we have to remember that if the ones digit is less than five we round down and if the ones digit is greater than five we round up. The final step is to calculate the two numbers that we’ve rounded. Amelia calculated that 270 add 150 are 420.

Let’s try an example: estimate the sum of 615 and 209 by rounding both numbers to the nearest 10. So we have to calculate the answer to 615 add 209 by estimating. That means we’re going to give an answer which is close to the real answer, but not exact. First we have to round both numbers to the nearest 10. First we need to round 615 to the nearest 10. If we use a number line, it helps us to see that 615 is halfway between 610 and 620. When we’re rounding, we look at the ones digit. If it’s five or more, we round up. 615 to the nearest 10 is 620. To round 209 to the nearest 10, we look at the ones digit, which is nine. This tells us we need to round up to 210. Now we’ve rounded our numbers, we have to calculate 620 plus 210. 600 plus 200 is 800. 20 plus 10 is 30. 800 and 30 is 830. So our estimate for the sum of 615 and 209 is 830.

Let’s try one more example: estimate the sum of 262 and 147 by rounding both numbers to the nearest 10. So we’re going to estimate 262 plus 147. First we need to round. We’ll start with our first number 262, and we can see that the ones digit is a two. Two is less than five, so we round down to 260. Now we need to round 147 to the nearest 10. The ones digit is a seven, so we need to round up to the next 10, which is 150. Now we can calculate 260 plus 150. 60 plus 50 is 110. 100 and 200 makes 300. 300 plus 110 makes 410. We estimated the sum of 262 and 147 to be 410.

In this video, we’ve learned how to estimate the sum of two three-digit numbers by first rounding to the nearest 10 and then calculating our answer.