A cistern has two inlets A and B which can fill it in 12 hours and 15 hours respectively. An outlet can empty the full cistern in 10 hours. If all the three pipes are opened together in the empty cistern, how much time will they take to fill the cistern completely?

Explanation

Inlet A's 1 minutes work=`1/12.`
Inlet B's 1 minutes work=`1/15`.
Outlet C's 1 minute work=1/10.
Inlet A and B and outlet C's 1 minute work=`1/12+1/15-1/10`
=`3/60`
=`1/20`.
The empty tank can be filled completely in =`20` hours.