Pipes and Cistern - General Questions

Two pipes A and B can fill a tank in 23 hours and 9 hours respectively. If both are opened together, how long will they take to fill the tank?

A pipe fills a tank in 4 hours. Because of a leak at the bottom it takes 8 hours. If the tank is full, in how much time will the leak empty it?

Two pipes A and B can fill a tank in 26 hours and 15 hours respectively. If both are opened together, how long will they take to fill the tank?

Pipes A and B can fill a tank in 15 and 16 minutes, and pipe C can empty it in 37 minutes. If all three are opened together, how long will it take to fill the tank?

A pipe fills a tank in 5 hours. Because of a leak at the bottom it takes 8 hours. If the tank is full, in how much time will the leak empty it?