A tank can be emptied by either of two pipes. one pipe used alone requires 6 minutes to empty the tank, and the other requires 4 minutes. how long will it take to empty the tank if both pipes are used at the same time.

IN one minute pipe 1 can empty in 1/6  th of tank.

In one minute pipe 2 can empty 1/4 th of tank.

In one minute if both pipes are used they can empty (1/6+1/4) th of tank

=(2+3)/12

=5/12.

It takes 12/5 minutes to empty the atbk if both pipes are used.

12/=2.4 minutes = 2M+ (0.4*60) =2 M 24 sec.