Video: Estimating Differences between Two-Digit Numbers

Look at how Anthony estimated the difference by rounding to the nearest ten. Estimate the difference by rounding to the nearest ten. 57 − 21 is about _, and 53 − 28 is about _.

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Video Transcript

Look at how Anthony estimated the difference by rounding to the nearest ten. 43 take away 31 round, use the number line. 40 take away 30 equals 10. 43 Take away 31 is about 10. Estimate the difference by rounding to the nearest ten. 57 take away 21. 57 take away 21 is about what? 53 take away 28. 53 take away 28 is about what?

We can see that Antony estimated the difference by rounding to the nearest ten. He rounded 43 to 40 and he rounded 31 to 30. Then, he subtracted 30 from 40 to give him his estimated answer of 10. So we know that 43 take away 31 is about 10. We have to use the same method to find 57 take away 21 and 53 take away 28. Let’s calculate 57 take away 21.

First, we have to round both numbers to the nearest ten. First, let’s find number 57. 57 is between the two tens 50 and 60. 55 is halfway between these two tens. 57 would go here on the number line. So the nearest ten to 57 is 60. 21 would go here on the number line and its nearest 10 is 20. Now we can calculate. Zero take away zero is zero and six tens take away two tens is four tens. 60 take away 20 is 40. So 57 take away 21 is about 40.

Next, we have to calculate 53 take away 28. But first, we need to round both of the numbers. Here’s number 53 and here’s number 28. The nearest ten to 53 is 50 and the nearest ten to 28 is 30. Now, we can calculate 50 take away 30. Zero take away zero leaves us with a zero and five tens take away three tens is two tens. So 53 take away 28 is about 20.

We used Anthony’s method to find the difference by rounding both numbers first to the nearest ten. We rounded 57 to 60 and 21 to 20. 57 take away 21 is about 40. And we rounded 53 to 50 and 28 to 30. 53 take away 28 is about 20. We use rounding to help us estimate the answers.

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