YSK - X percent of Y is equal to Y percent of X. So, if you want to find out what 7% of 50 is, you could instead find out what 50% of 7 is, which is 3.5. This means that 7% of 50 is also equal to 3.5.
Percentage is just a fancy form of decimal point. I just multiply them together then move decimal digit to the left twice.
That's why Free Software movement should be added to textbooks.