1. Essential formulas for pipes and cisternsthe time taken to fill the entire tank = (xy) / (y+x). If two pipes fill a tank with water in x and y hours, respectively, and a third pipe is opened to empty the tank in z hours, the time taken to fill the tank = 1 / (1/x)+(1/y)+(1/z), and.

2. A pipe can fill a tank in a hrs. Due to a leak in the bottom it is filled in b hrs. If the tank is full, the time taken by the leak to empty the tank = ab/(b - a) hrs.

3. The time taken to fill the entire tank = (xy) / (y+x). If two pipes fill a tank with water in x and y hours, respectively, and a third pipe is opened to empty the tank in z hours, the time taken to fill the tank = 1 / (1/x)+(1/y)+(1/z), and.