Percents - Problem 1 15,221 views

Teacher/Instructor Alissa Fong
Alissa Fong

MA, Stanford University
Teaching in the San Francisco Bay Area

Alissa is currently a teacher in the San Francisco Bay Area and Brightstorm users love her clear, concise explanations of tough concepts

How to solve "What is x percent of...?" A strategy to solve percent problems is to use the proportion: is/of = %/100 ("is" over "of" equals "percent" over "100"). Fill in the known values from the given problem, and use a variable, such as x, for the unknown value. The unknown value in this kind of problem is the "is" value. Cross multiply to solve. To do this, multiply the numerator of the first fraction by the denominator of the second fraction and set it equal to the denominator of the first fraction times the numerator of the second fraction (is * 100 = of * %). Then solve for the unknown value (the variable).

Whenever you see percent problems a strategy that might help you is thinking of is over of equals % over 100. That's a proportion that you can easily cross multiply to solve. Let's look at this one. What is 85% of 20?

Well let's start with this, what is. That tells me my is value, I'm going to call x, I don't know what x stands for, what I don't know. 85 is my percent number so it's going to go there. I always have 100 in that second fraction percent over 100 and then of 20, my of number is 20. Boom guys, that's it. It's set up as a proportion. From there you guys could cross multiply if you know how to do that or if you're not sure how to do that you can multiply both sides of this equation by 20 so that x would be isolated.

I'm going to show you how to cross multiply. If you cross multiply that means 100x, that's where I got it, those are the two diagonals is equal to 85 times 20. That's how you set up cross multiplying. Grab yourself a calculator and figure out what 85 times 20 is. I'll do it really quickly here, 85 times 20 is 1700. So I have 100x equals 1700, divide both sides by 100 and you'll get that x is equal to 17. That's it. What number is 85% of 20? Well, 17 is 85% of 20.

Make sure your answer makes sense also. 85% is like less than the whole thing, right? 100% would be 20, 85% should be less than 20 so it makes sense that I got the answer 17. Whenever you're doing Math, it's a good idea to check at the end and make sure your answer makes sense.