12. 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 minutes, C is also opened. In how much time, the tank is full?
Answer:
Part filled in min. = 7[1/36 + 1/45] = 7/20
Remaining part = [1-7/20] = 13/20
Net part filled in 1 min. when A,B and C are opened = [1/36 + 1/45 - 1/30] = 1/60
Now, 1/60 part is filled in 1 min.
13/20 part is filled in [60 * 13/20] = 39 min.
Therefore, Total time taken to fill the tank = (39+7)min. => 46 min.
