By Vince Kotchian. Full-time GRE tutor since 2007, author of Barron's 6 GRE Practice Tests, three perfect 170 verbal scores on the real GRE, and moderator of r/GRE.
Percent change is a topic that often confuses people, but it's not hard if explained the right way.
The question is asking you to calculate the percent change between two numbers. All you have to do is find the difference between the two numbers. Then, take that amount, and divide it by the original number, and multiply your answer by 100 to convert it to a percent. Boom. Done.
The tricky part comes in if you don't know which number was the original one - i.e., if there's no chronology present (like yesterday's high and today's high).