Two water pumps, working simultaneously at their respective constant rates, took exactly nine hours to fill a certain swimming pool. If the constant rate of one pump was 1.5 times the constant rate of the other, how many hours would it have taken the faster pump to fill the pool if it had worked alone at its constant rate? Choose the correct answer
A) 12  B) 14  C) 15  D) 16  E) 18
Let x be the rate of the slower pump. Then 1.5x is the rate of the faster pump.   Both pumps together can fill 1/6 of the pool each hour.   2.5x = 1/9   x = 1/22.5   1.5x = 1.5/22.5 = 1/15   The faster pump could fill the pool in 15 hours.   The answer is 15.
<result>C</result>
Two water pumps, working simultaneously at their respective constant rates, took exactly nine hours to fill a certain swimming pool. If the constant rate of one pump was 1.5 times the constant rate of the other, how many hours would it have taken the faster pump to fill the pool if it had worked alone at its constant rate?