Solving Indirect Proportion Math Problems

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Indirect proportion problems sound technical, but it helps to think of the word "indirect" as "inverse." Discover why an inverse has the unknown, X, in the denominator with help from a math teacher in this free video on proportions in math.

Part of the Video Series: Fractions & Proportions
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So how does one solve indirect proportion math problems? Hi, I'm Jimmy Chang. I've been teaching college mathematics for nine years, and, indirect proportion problems sounds very technical, but as long as you are able to see what kind of problems they are, you might have solved them before, maybe just under a different name. Now, indirect proportion problems, the key word here is indirect. Now indirect is actually another way of saying the word 'inverse'. Now, whenever you have the word inverse, that means that the unknown, let's just call it X, X will be in the denominator. So, when that happens, you know that there's going to be some solving going on, and because X is the denominator it makes it a little bit difficult, but it doesn't have to be as long as you know how to solve proportion problems. So here's a quick example for you. Suppose you have three over four equals to seven over X. This is an example of an indirect proportion problem because you have an X in the denominator. But, if you know how to solve proportion problems, you'll know that the strategy here is to cross multiply. So, as we've been doing before, and perhaps you've learned this in other areas, if you cross multiply, you take the denominator and then you multiply it with the numerator on the other side. So that means you have four times seven, which is going to give you twenty-eight. On the other side, you take the denominator, which is X, and you multiply it with the other numerator on the other side, so X times three is going to give you three X. And so all of a sudden your fraction equation, it's not so tough any more. Now to solve this, all you need to do, of course, is divide both sides by three. So, as a result, the threes cancel. The three doesn't quite cancel with the twenty-eight, so it stays where it is. So as a result you have X is equal to twenty-eight thirds. And so you have a solution. So, once you know what an inverse proportion, which is related to indirect proportions, it's smooth sailing from there. I'm Jimmy Chang, and that's exactly how you solve indirect proportion math problems.

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