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Lesson 12: Rounding Numbers to the Nearest Ten
In math we are often asked to estimate sums, and this is also something that we do in real life. For example, in the supermarket, we might estimate how much our total bill will be. One way to estimate sums is by first rounding each number that we are adding. In this lesson, you'll learn how to round numbers to the nearest ten.
Don't forget to watch the embedded video clip for this lesson at the bottom of the page. Please be sure to read the embedded video information and disclaimer.
We round numbers when we want to have an easier time adding, and we're not concerned about having an answer that is exactly correct. For example, if I asked you to add 19 + 32, you would know that the answer was close to 50. This is because 19 is about the same as 20, and 32 is about the same as 30, and 20 + 30 = 50. Of course 50 is not the exact answer, but if you just wanted to have a rough idea, 50 would be about right.
Here's how to round a number to the nearest 10. This procedure must be memorized. Look at the rightmost digit of the number you want to round--the digit in the ones place. If that digit is 5 or above, round the number up to the next highest multiple of 10. For example, 35 rounds up to 40. So does 36, 37, 38, and 39.
If the rightmost digit is 4 or below, you would round down to the next lowest multiple of ten. As another example, if your number is 41, you would round down to 40. You would also round 42, 43, and 44 down to 40. If you need to round a number that ends with 0, there is nothing to do since it's already a multiple of ten.
This works for bigger numbers also. 387 would round up to 390. The last digit is 5 or above, so we round up to the next highest ten. 923 would round down to 920, since the last digit is 4 or below.
Be careful of tricky problems. 397 would round up to 400. The last digit is 5 or higher, so we round up, but the next higher ten actually takes us to 400. Make sure that you see why. We have to increase the digit in the tens place, and since it's already 9, we must make it a 0, and then increase the digit in the hundreds place.
In this lesson we rounded numbers to the nearest ten. Later you'll be asked to round numbers in different ways. For example, if you had to round the number 3,924 to the nearest thousand, it would be 4,000, since it's closer to 4,000 than to 3,000. You'll learn more about different types of rounding in later lessons.