There is multiple ways to answer a problem like this, but I will show you one. First, you must see how many times the total number will go onto 100. In this case, our total number is 50. 50 times 2 is 100. Then, we multiply the number we are trying to make a percentage. That number is 9, because she gave 9 dollars. We must multiply 9 by the same number we did to 50. 9 times 2 is 18, therefore the answer is 18%.