When should I implement the two different percent increase strategies? When should I use the formula of taking the differences between the two numbers, then dividing by the original. And when should I take a number and divide by the other number to see the percent increase (as in this example).
It all comes down to the wording of these problems, which can be admittedly confusing. There are a lot of different ways to word the questions, but we can break them down into two categories:
The original amount is contained in the result: look for the phrase "is what percent of". Here, you simply use new/old * 100
Car sales increased from 500 in July to 600 in August. August car sales are what percent of July car sales? 600/500 * 100 = 120%
The second category is probably a bit more common.
Comparing the difference to the original: look for the phrase "is what percent greater/less than". Here, use the formula (new-old)/old * 100
Car sales increased from 500 in July to 600 in August. August car sales are what percent greater than July car sales? (600-500)/500 * 100 = 20%