How to do a helppppppp!?

Tom is asked to go buy blue, red and orange balloons from a store. When, he gets to the store, he forgets how many of each colour he is should to buy. He can remember he is told to buy a total of 100 balloons and the total number of blue ones should be three times of the total number of red ones. He is given an exact amount of money $745 for them. He also knows that the blue, red and orange balloons cost $9, $4 and $7 each, respectively. How many balloons of each colour was he asked to buy?