Percents - Problem 2 11,655 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 percent of x is...?" 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 "percent" 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).

This problem says, what percent of 108 is 9. Before I do this problem I'm going to estimate what my answer might look like by thinking about what 10% of 108 would be. 10% of 108 means, move a decimal place over, 10% would be 10.8. So I'm looking for 9, what percent is 9. 9 is going to be around 10%, a little bit less than 10%. Before I do any Math I'm going to think what my answer might be. Now let's do it using the proportion is over of equals percent number on top of 100.

Well let's see. What percent, that means I don't know my percent value. I'm going to make that x but it's still going to be on top of 100. My is number, there it is, is 9 and of is 108. Okay not so bad. That's a proportion that I can solve by either cross multiplying or multiplying both sides by 100. I'm going to show you how you'd multiply both sides by 100 because now those 100s cancel out on that side and I have just plain old x, x is equal to 900 divided 108. Grab yourself a calculator. If you do 900 divided 108, you'll see that x is equal 8.3 repeating percent. 8.3, I'm going to round it like that. That makes sense. I thought my answer should be a little less than 10 and it's 8.3. Pretty good, that makes me feel good because I estimated pretty accurately.