13. Two pipes X and Y can fill a cistern in 24 min and 32 min respectively. If both the pipes are opened together, then after how much time Y should be closed so that the tank is full in 18 minutes>

14. A cistern has a leak which would empty it in 8 hours. A tap is turned on which admits 6 litres a minute into the cistern and it is now emptied in 12 hours. How many litres does the cistern hold?

15. Two pipes A and B can fill a tank in 15 hours and 20 hours respectively while a third pipe C can empty the full tank in 25 hours. All the three pipes are opened in the beginning. After 10 hours, C is closed. The time taken to fill the tank is: