1.8 Rounding Decimals

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Math Basics  >  Unit 1 Number Sense  >  Lesson 1.8 Rounding Decimals

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In this lesson we're going to learn how to round decimals. In our previous lesson, we learned how to round whole numbers and the steps are pretty much the same. We find the place value that we want to focus on, look to the neighbor and then the neighbor tells us what our place value should do. But with decimals, we do need to pay extra careful attention to what place value we're being told that we need to round to. So if you don't already remember all of your place values for decimals, make sure that you get some practice with those. Let's look at our first example. We're going to round this number to the thousandth place. Now remember, whenever the place value name ends with th at the end, that's a big clue that our place value is after the decimal point. So our digit that's in the thousandths place is the eight. So now we need to figure out what that eight is going to do. So we asked the neighbor, which is the three, since three is less than five, then our eight will stay the same. Anything to the left of it, all those digits in the front, 517.96, none of those digits are affected. So let's just copy those over. And now we know that our eight is going to stay the same. And then anything after it becomes a zero. So that three would become a zero just like we learned with whole numbers. Now with decimals, whenever you have a zero at the end, we don't have to necessarily write it in. So we can just get rid of that zero that's at the end and make our final answer 517.968. Or we can read it as 517 and 968 thousandths. But the important thing when we're rounding decimals is to make sure that the last digit is in whatever place value we want to round to. So since we're rounding to the thousandths place, we want the last digit to be in the thousandths place. So that's why we were able to leave that zero off at the end as well because we only really needed to stop at the 8. Now let's look at what happens when we round to the digit that's in the hundredths place. We look to the neighbor, the eight, to see what that six will do. Since the eight is in the five or higher category, that tells us that the six is going to round up and become a seven. Remember, all the digits in front of that though will stay the same. So let's copy those over. Now the six will round up to a seven and everything after becomes a zero. Just like we mentioned before though, we don't really need to leave all those zeros at the end, so we can knock those off. Now we have 517 and 97 hundredths. As long as the last digit in our final answer is in the place value that we're rounding to, in this case the hundredths, then we're good to go. Let's look at one more example. This time we're going to round to the tenth place. Our digit that's in the tenth place is a nine. We look to the neighbor, the six. The six is going to tell us that our nine is going to round up. Now, if we round nine up, it becomes ten. But we can't fit a one and a zero in that spot. We can only fit one digit in that place value. So that means that the one from our ten is going to carry over to the seven. And now the seven is going to become an eight. So now we have 518.0. Let's take a look at what just happened there. The nine needed to round up, so it became a ten. But the one from the ten got carried over to the seven. That made the seven become an eight. And then the zero from our ten was what is left in that position where the nine was. Now, just like we've been doing, everything after can become a zero. And just like with any decimal, any zeros at the end can be knocked off. But remember, since we need to round this number to the tenth place, we need to still have a digit left in the tenths place position. So we still need to keep the zero that's in the tenths place. But we can knock off the other zeros after that. So now our answer is 518.0. And yes, this is technically a whole number, 518. But because they asked us to round to the tenths place, we need to make sure that our last digit is in the tenths place. So that's why we have to keep that zero at the end there. So, just to review, when we're rounding decimals, we round to the place value that the directions tell us to do. And we need to make sure that the last digit is in that place value. And then any zeros after that, we can just drop them off. Now you know how to round decimals.

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