One pipe can fill a pool three times faster than a second pipe. When both pipes are opened, they fill the pool in seven hours. How long would it take to fill the pool if only the slower pipe is used?
Choices:  A) 10 hr  B) 12 hr  C) 15 hr  D) 20 hr  E) 28 hr
fast pipe + slow pipe = together   One pipe fills the pool three times faster than the other pipe so,   hourly fill rates:   1/x + 1/3x = 1/7   Let x = number of hours it would take the fast pipe to fill the pool thus,   x=9.3 hours   9.3*3 = 28 hours   Answer: 28 hr
<result>E</result>
One pipe can fill a pool three times faster than a second pipe. When both pipes are opened, they fill the pool in seven hours. How long would it take to fill the pool if only the slower pipe is used?