Pipes and Cistern - General Questions

Two pipes A and B can fill a tank in 16 hours and 21 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?

Two pipes A and B can fill a tank in 7 hours and 27 hours respectively. If both are opened together, how long will they take to fill the tank?

Two pipes A and B can fill a tank in 15 hours and 22 hours respectively. If both are opened together, how long will they take to fill the tank?

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?