7th Grade Math Help: Adding and Subtracting Decimals

You've most likely mastered adding and subtracting decimals already, but you'll continue to use this skill in algebra and geometry. Keep reading for a refresher on this concept, as well as an overview of its applications for 7th grade math.

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Addition and Subtraction of Decimals

When you're adding and subtracting decimals, you'll use the same process that you use for whole numbers. You just need to make sure that you've lined up the decimal points of the numbers that you're working with. You can add zeros to one of the decimals if necessary.


In 7th grade geometry, you'll use decimals in lots of problems, including ones that involve angle measures. For example, you might be told that Angle A measures 87.5 degrees and you must find its supplementary angle. Since two supplementary angles add up to 180 degrees, you'll solve this problem by subtracting 87.5 from 180. You'll need to change 180 to 180.0 so that the decimal points can be lined up. The solution to 180.0 - 87.5 is 92.5.


You'll also add and subtract decimals when you're solving algebra equations and word problems, like this one:

'Franco ran 3.5 kilometers on Saturday, and he ran a total of seven kilometers over the whole weekend. How many kilometers did he run on Sunday? Write an equation to represent this problem and then solve it.'

First, you might write an equation like 3.5 + x = 7 to represent the problem. To find the value of x, you'll need to subtract 3.5 from seven. Since you need to line up the decimal points, don't forget to write seven as 7.0. Your final answer will be x = 3.5 because 7.0 - 3.5 = 3.5. This means Franco ran 3.5 kilometers on Sunday.

You might also encounter algebra equations like x - 7.265 = -10.2. To find the value of x, you'll need to add 7.265 to both sides of the equation. Since 7.265 has more decimal places than -10.2, you'll need to add two zeros to this number to fill in the additional decimal places, like this: -10.200. Now, both decimals extend to the thousandths place.

You'll also need to pay attention to the signs on each number. You'll be adding a smaller positive number to a larger negative number, which means your answer will be negative: -10.200 + 7.265 = -2.935. Likewise, if you add a larger positive number to a smaller negative number, your answer will be positive.

Tip: If you're adding or subtracting decimals on a calculator, you usually don't need to fill in the extra zeros because the calculator will do this automatically. For instance, you could just enter -10.2 + 7.265 instead of -10.200 + 7.265. However, you must fill them in if you're performing the operation by hand.

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