Rounding Decimals & Finding the Missing Digit

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  • 0:01 Decimal Numbers
  • 1:00 Rounding Decimals
  • 3:37 Finding a Missing Digit
  • 4:57 Example
  • 5:53 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 watching this video lesson, you will learn how to round decimals and how to use your knowledge of rounding to help you find a missing digit. Learn what the difference is between rounding decimals and rounding regular numbers.

Decimal Numbers

In math, we come across our regular numbers, such as 1, 2, and 3, very often. But as you keep learning more and more math, you will come across more and more decimal numbers, numbers with a decimal point. Scientists and other professional people also deal with decimal numbers on an almost daily basis.

Examples of the kinds of decimal numbers that you will see include 12.45, 0.001, and 0.00457. If you spend any time shopping, you will see that most prices in stores are decimal numbers. You hardly ever see something priced at $13, but you will see the same thing priced at $12.99 or something similar. In this video lesson, we look at rounding our decimal numbers.

Rounding Decimals

When you are shopping, it can be very useful to round your decimal number to a whole number. This helps you to easily add up your total. For example, $12.99 rounded to a whole number becomes $13. Don't you think $13 is a lot easier to work with than the $12.99?

How do we round our decimal number? We follow the same rule as we did with regular numbers. The only difference is that we have a decimal point to consider. We just have to make sure that our decimal point stays in the same place and that it doesn't move when we round. So, to round $12.99 to the nearest whole number, we are essentially rounding to the ones place.

What digit is in the ones place? It is the 2. To figure out whether we should round up or round down, we look at the digit to the right of the ones place, the 9. Is this digit 5 or greater? If so, we round up. If this digit is less than 5, then we round down by keeping the digit that we are rounding and changing all the digits to the right of that digit to 0.

In our case, the digit to the right of the 2 is a 9, which is greater than 5, so we round our 2 up to 3 to make $13. If our number was $12.43, then we would round down by keeping the 2 a 2 and changing the 4 and 3 to zeroes to make $12.

We can also round our decimal point to any digit to the right of the decimal point. For example, we could round the number 0.123456 to three decimal spaces. What is the digit in the third decimal space? It is the 3.

We now need to look at the digit directly to the right of this 3. What digit is that? It's the 4. 4 is less than 5, so that means we round down by keeping our 3 and changing all the digits to the right of the 3 to zeroes.

Our rounded decimal number then becomes 0.123. Because the end zeroes are to the right of the decimal point, we don't write them down. If the zeroes were to the left of the decimal point, then we would write them down.

Finding a Missing Digit

Sometimes in math you will be asked to find the missing digit of a number. We can use our knowledge of rounding to help us answer these types of questions. For example, you might see a problem like this:

'Find the missing digit in 0.00?34, if the rounded number is 0.002.'

In this problem, we see that the third decimal space is our missing digit and that we are rounding to three decimal spaces. What number could this be?

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