One pipe fills a storage pool in 3 hours. A second pipe fills the same pool in 3 hours. When a third pipe is added and all three are used to fill the pool, it takes only 1 hour. Find how long it takes the third pipe to do the job alone.

Let's follow this:

1/3 + 1/3 + 1/x = 1/1

x will be equal to 3.

Also 3 hours. The first fills 1/3 each hour just like the second one. So for one hour both of them together fill 2/3 of the pool. The third pipe fills the rest 1/3. So if it fills 1/3 of the pool each hour, alone it will do the job for 3 hours.