Math: percents: multiply or divide

Tutoring math, percents come up. The tutor mentions a couple of quick methods.

In math, “of” typically means “multiply:” thus, 37% of 165 is (37/100)x165 = 61.05.

On the other hand, if the total isn’t known, but a certain percent is, one can divide. For instance, if one is asked “75% of what number is 130?”, a quick way to proceed is 130/(75/100) = 173.33. Note the position of the brackets: 75% must be converted to 0.75 before the calculation.