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.

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.

Let's follow this:
1/3 +1/3 + 1/x =1/1
x will be equal to 3.

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