Two pipes A and B can fill a tank in 36 min. and 45 min. respectively. A water pipe C can empty the tank in 30 min. First A and B are opened. after 7 min,C is also opened. In how much time, the tank is full?

      Part filled in 7 min. = 7*((1/36)+(1/45))=(7/20).

      Remaining part=(1-(7/20))=(13/20).

     Net part filled in 1min. when A,B and C are opened=(1/36)+(1/45)-(1/30)=(1/60).

     Now,(1/60) part is filled in one minute.

     (13/20) part is filled in (60*(13/20))=39 minutes.