How to Set Up Proportion Problems

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Setting up a proportion problem is fairly simple once it is understood that they can be set up as fraction equations. Be consistent with both side of a proportion or fraction equation 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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Video Transcript

So, how does one set up proportion problems? Hi, I'm Jimmy Chang, and I've been teaching college mathematics for nine years. And whenever you see the phrase proportion problems the idea of fraction problems should immediately come to mind. Now, I know fraction equations aren't exactly our favorites, but once you know exactly what it is you're comparing you should be able to set it up in a fairly straightforward fashion as long as you're consistent about which side goes with which. So, here is an example as to what we're going to be doing. Suppose you have the problem three gallons for sixty one miles. In other words, three gallons will get you to travel sixty one miles. How many gallons will it take to travel three hundred miles? Now, what you have to make it to do is to make a decision as to what number you want on top, which one you want on the bottom. Now, being that gallons is mentioned first and miles is mentioned second what you can do is make a comparison of gallons over miles now and just putting the numbers where they belong. So, the first comparison is three gallons so the three would be on top and sixty one would be at the bottom. Now, to figure out how many gallons for three hundred miles all you need to do is have an equal sign and then have another fraction on the other side. Now, being that we're trying to figure out how many gallons, remember gallons is in the numerator so let's call that x, and then for three hundred miles because you know miles is is in the denominator you put three hundred at the bottom. Now, if you wanted to you could have set miles be the numerator and then gallons in the denominator so you would have something that goes like this and you'll have miles which was sixty one in the first comparison over three in the first comparison for gallons equal to. And then this time, how many gallons is the denominator you have x and then miles for three hundred. Now, does it matter which comparison you use? No, but you just have to be consistent with whatever decision that you make for both sides of the equation. So, I'm Jimmy, and that's an example as to how you set up proportion problems.

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