Question

one pipe can fill a pool 1.25x faster than the second pipe. when both pipes are open they fill the pool in 5 hours. how long would it take to fill the pool if only the slower is used?

Answer 1

Suppose the bigger time fills up the pool in x hours, the smaller pipe fills up the pool in 1.25x. 
each hour the pipe finishes 1/x portion of the job, and the smaller pipe finishes 1/1.25x portion
in 5 hours, with both pipe working, the pool is full, that is 5/x + 5/1.25x =1
5/x +4/x=1
x=9
1.25*9=11.25=11 and a quarter=11 hours and 15 minutes
it will take the slower pipe 11 hours and 15 minutes to fill up the pool.