A cistern with three pipes
A cistern has three pipes. The first two pipes separately can fill the empty cistern by water in 10 and 20 hours respectively. The third pipe can only flows out water from the cistern.
If all of the pipes of that cistern are opened, it takes 12 hours to fill the empty cistern.
Find the time for the third pipe to vacate the filled cistern, if the first two pipes became closed?