Pipes and Cistern - General Questions

Pipes A and B can fill a tank in 29 and 30 minutes, and pipe C can empty it in 28 minutes. If all three are opened together, how long will it take to fill the tank?

Pipes A and B can fill a tank in 11 and 8 minutes, and pipe C can empty it in 21 minutes. If all three are opened together, how long will it take to fill the tank?

A pipe can fill a tank in 14 hours and another pipe can empty it in 18 hours. If both are opened together, in how much time will the tank be filled?

Two pipes A and B can fill a tank in 18 hours and 12 hours respectively. If both are opened together, how long will they take to fill the tank?

A pipe can fill a tank in 15 hours and another pipe can empty it in 40 hours. If both are opened together, in how much time will the tank be filled?