A % is out of a hundred, so you don't need the %/100. Then, you wouldn't do cross multiplication, you would just divide 15/27. Remember, it would be 15/27 because it's percent markup, not the percent the price increased by. So, doing that simple calculation on a calculator you get 0.5555. This converted to a percent is just moving the decimal to the right twice, or multiplied by a 100. That would give you 55.55%, and that's your answer. Make sense?