Question

One pipe fills a storage pool in 8 hours. A second pipe fills the same pool in 4 hours. When a third pipe is added and all three are used to fill the​ pool, it takes only 2 hours. Find how long it takes the third pipe to do the job

Answer 1

The first pipe fills 1/8 of a pool in one hr.
The second pipe fills 1/4 of a pool in one hr.

(1/8+1/4)2= 3/4 done by the first two pipes in 2 hours.
Therefore the last pipe must do 1 1/4 of the filling in 2 hours, which is 5/8 per hour.

5/8x=1
x=8/5

It takes the third pipe 1 hour and 36 minutes to fill it by itself.