First they have to have a common denominator. To do that, just simple find what least number when multiple by a number does, 9, 4, and 5 go into, which each number has a multiple of 180(this will be the new denominator for each fraction , and also your answer). Then you will basically divide 180 by each of those numbers, to get the number you need to times the top numbers to. Then you are able to add the new top numbers together and get your answer.