One pipe can fill a pool 1.25 times faster than a second pipe. When both pipes are opened, they fill the pool in 4 hours. How long would it take to fill the pool if only the slower pipe is used?	Options:	A) 11.25	B) 11.52	C) 1.25	D) 9	E) 7.2

Rationale: "say the rate of the slower pipe is r pool / hour , then the rate of the faster pipe would be 1.25 r = 5 r / 4 . since when both pipes are opened , they fill the pool in four hours , then their combined rate is 1 / 4 pool / hour . thus we have that r + 5 r / 4 = 1 / 4 - - > r = 1 / 9 pool / hour - - > time is reciprocal of rate thus it ' s 9 / 1 = 9 hours . answer : d ."
Answer: D