When you can't remember even a basic equation like the one listed below, make up your own much simpler problem and see if you can deduce the underlying relationship between the simpler numbers in order to apply your understanding to the actual problem.

For example, "2 is what percent of 4?" Well, 2 is half of 4 (2 out of 4 is equivalent to 1 out of 2), so 2 is 50% of 4. If you "see" that 2/4 = 1/2 = 0.50 = 50% then you understand the underlying relationship between the 2 and 4 in this particular problem
and can now extrapolate to the actual problem.

Now, how must the 72 be related to the 480 in the original problem? As a fraction(72/480). Divide and conquer!

72 divided by 480 = 0.15 = 15/100 = 15%.