math
posted by sam .
One pipe fills a storage pool in 20 hours. A second pipe fills the same pool in 15 hours. when a third pipe is added and all three are used to fill the pool , it takes only 6 hours. Find how long it takes the third pipe todo the job.

If the 3rd pipe takes x hours,
1/6 = 1/20 + 1/15 + 1/x
x = 20