Question 589726
One pipe fills a storage pool in 12 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 2 hours.
 Find how long it takes the third pipe to do the job. 
:
Let x = time required by the 3rd pipe
Let the completed job = 1; (a full tank)
:
{{{2/12}}} + {{{2/3}}} + {{{2/x}}} = 1
Multiplying by 12x will clear the denominators