PUZZLEVILLA

2 pipes A and B can fill a tank in 20 and 30 minutes respectively. If both pipes are used together, how long will it take to fill the tank ?

A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9 hours. If both the taps are opened together, after how much time will the cistern be filled ?

Pipe A and B together can fill a tank in 6 hours. If B takes 5 hours more than A to fill the tank, what is the time taken by A?

2 pipes A and B can fill a cistern in 60 minutes and 75 minutes respectively. There is a third pipe that empties the cistern. If all 3 pipes are simultaneously opened, cistern is full in 50 minutes. In how much time can the third pipe empty the cistern ?