Pipes and Cistern - General Questions

Two pipes A and B can fill a tank in 13 hours and 11 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 10 and 26 minutes, and pipe C can empty it in 17 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 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?

Two pipes A and B can fill a tank in 29 hours and 16 hours respectively. If both are opened together, how long will they take to fill the tank?

A pipe can fill a tank in 19 hours and another pipe can empty it in 20 hours. If both are opened together, in how much time will the tank be filled?