Estimating a Difference by Rounding

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  • 0:01 Rounding Numbers Can…
  • 2:04 Example 1
  • 3:28 Example 2
  • 4:18 Lesson Summary
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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

After studying this lesson, you will understand rounding basics. You'll also learn how to quickly solve subtraction problems by using your rounding skills.

Rounding Numbers Can Be Helpful

In math and in real life, you will come across many problems where you need to find the difference. For instance, when you go shopping and you use a coupon for $10 off your purchase of $50 or more, in order to find out your total amount due, you need to find the difference. You need to subtract the $10 from the amount that you are purchasing. So difference problems are really just subtraction problems.

Many times, you do want to be exact and use the numbers you are given to find an exact answer, but sometimes, rounding, replacing a number with an approximate simpler number, can give you a general idea of what kind of exact answer to get, a so-called rough estimate. This helps you a lot when you are out shopping. If you can round your numbers and quickly get an estimate of your exact answer, you won't be surprised at the register when the final purchase price is given to you. You will already know that it will be close to your estimated - your rounded - answer.

Do you remember how to round? You pick how much you want to round to. You can pick tens, hundreds, thousands, and so on. Or you can pick to a certain decimal point, like two decimal points, three decimal points and so on. Either way, once you have chosen how much to round to, you will then look at the number directly to the right of the digit you chose to round to. If it is 5 or higher, then you round your digit up, and if it is less than 5, then you round down by keeping the digit you are rounding and bringing all the numbers to the right of that digit to 0.

For example, rounding the number 16 to the tens place gives you 20. You round up because the number directly to the right of the digit that you are rounding - the 6, which is directly to the right of the 1 in the tens place - is 5 or greater. The 1 in the tens place changes to a 2. If our number is 13, then we round down by keeping our tens digit, the 1, and bringing the 3 down to get 0. We get 10.

Let's look at a couple of examples.

Example 1

You are shopping at the store where you have the $10 off coupon for purchases of $50 or more. Right now, all the things you have in your cart add up to $87.24. Use rounding to help you estimate what kind of a total to expect at the register.

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