The Educational Testing Service has cleverly figured out that students are not sufficiently familiar with the topic of percent of change. Consider this simple problem:
A store owner raises the price of an item from $40 to $50. By what percent did he increase the price? There is a formula that every student must know for the SAT:
Percent of Change = (Change/Original) * 100
In the case of this problem, the price increased by $10, so that’s the change. The classic question that students always have trouble with is whether to put that $10 over the $40 or over the $50. It’s always over the ORIGINAL. Thus, we put 10/40 = .25 * 100 giving us 25%. Then, there’s an interesting follow-up that has appeared on past SAT tests. Suppose this store owner now decides to be nice and bring the price back down to $40. Well, if he had raised the price by 25% surely he must now be lowering the price by the same 25%, right? Wrong!!! Let’s go back to our formula. He is now dropping the price by the same $10 but now the “original” price at the start of the new problem is $50. Therefore, we must put 10/50 this time, giving us .2 * 100, or a 20% decrease in price. Fascinating!
Tags: SAT Math, SAT Prep, SAT Preparation, SAT Test
Marty Rafson wrote the ESC math curriculum and has been an SAT math teacher, tutor, and curriculum developer for 30 years. He has been a high school math teacher for 36 years and a math department chairman for 25 years. He was also an adjunct professor at City College of New York School of Education.
