If Mrs. Wong had a whole package of flour (1, or 8/8), and she used 3/8 of it, how much is left over?
Find that answer, and then convert it to a percent. I think that you should memorize the conversion between certain fractions and percents, like fractions with an 8 as the denominator. But if don't have them memorized at the moment, then set up a proportion.
For example, if I want to convert 1/8 to percent, I would write
and solve for x by cross-multiplying:
So 1/8 = 12.5%.