Eduqo
Quantitative Aptitude›Ch 3: Time, Work & Distance›Pipes and Cisterns
Master inlet, outlet and leakage problems using the LCM method.
Treat inlet pipes as positive work rates and outlet pipes as negative work rates. Find total tank capacity using LCM of individual times, then combine net efficiency to find the answer.
Quick Memory Hooks
| Pipe Type | Time Taken | Efficiency / Hour |
|---|---|---|
| Pipe A (Inlet) | 10 hrs | +6 units |
| Pipe B (Inlet) | 15 hrs | +4 units |
| Pipe C (Outlet) | 12 hrs | -5 units |
Inlet Pipe
(Fills tank up)
Fills tank up
Adds water continuously at a fixed rate.
Outlet Pipe
(Empties tank out)
Empties tank out
Removes water from the tank.
Leakage
(Hidden outlet)
Hidden outlet
Acts exactly like an outlet pipe.
Net Work
(Combined effect)
Combined effect
Sum of all inlet and outlet efficiencies.
Adding outlet efficiency instead of subtractingSubtracting outlet efficiency from total work
Outlets reduce water, so their work rate must always be subtracted.
Ignoring leakage time when finding empty tank timeIncluding leak as negative pipe in LCM calculation
A leak is fundamentally an outlet pipe; treat it with a negative sign.
Assuming all pipes work together for full durationCarefully tracking opening and closing schedules
Read questions carefully to see if pipes are closed midway.
Q1.Pipe A can fill a tank in 12 hours and Pipe B can empty it in 15 hours. If both are opened together, in how many hours will the empty tank be filled?
Q2.An inlet pipe fills a cistern in 6 hours. Due to a leak at the bottom, it takes 8 hours to fill. If the cistern is full, how long will the leak take to empty it?