What is the formula to calculate the time taken to fill a cistern using multiple pipes?

The formula to find the total time taken to fill a cistern using multiple pipes is: where r₁, r₂, ..., rₙ are the rates of each pipe.

If Pipe A can fill a cistern in 4 hours and Pipe B can fill it in 6 hours, how long will it take for both pipes to fill the cistern together?

A cistern is filled by Pipe A in 8 hours and emptied by Pipe B in 12 hours. What is the net rate when both pipes are open?

If a cistern can be filled by Pipe A in 10 hours and emptied by Pipe B in 20 hours, how long will it take to fill the cistern with both pipes open?

How do you calculate the time taken to fill a cistern if one pipe is filling it and another is emptying it?

A cistern is filled by Pipe A in 5 hours and Pipe B fills it in 3 hours. If both pipes are opened for 1 hour, how much of the cistern is filled?

If Pipe A can fill a cistern in 6 hours and Pipe B in 9 hours, how long will it take both pipes to fill the cistern if they work together?

A cistern is being filled by two pipes, A and B, which can fill it in 4 hours and 6 hours, respectively. If both pipes are opened for 2 hours, how much of the cistern is filled?

If Pipe A can fill a cistern in 8 hours and Pipe B can empty it in 10 hours, how long will it take to fill the cistern with both pipes open?

A cistern can be filled by Pipe A in 15 hours and emptied by Pipe B in 10 hours. What is the net rate if both are open?

If Pipe A fills a cistern in 7 hours and Pipe B fills it in 14 hours, how long will they take together?

A cistern is filled by Pipe A in 5 hours and emptied by Pipe B in 15 hours. If both are open, how much of the cistern is filled in 1 hour?

If Pipe A can fill a tank in 9 hours and Pipe B can fill it in 6 hours, how long will it take to fill the tank if both pipes are opened for 3 hours?

A cistern can be filled by Pipe A in 12 hours and emptied by Pipe B in 18 hours. How long will it take to fill the cistern with both pipes open?