Pipes and Cisterns

Time and Work applied to filling/emptying tanks — inlet pipes add efficiency, outlet pipes subtract it.

📖 ~4 min read 📐 RRB NTPC / Group D — Mathematics

Core Approach

1. Capacity = LCM of all given times.
2. Inlet pipe efficiency = +Capacity/its time. Outlet pipe efficiency = −Capacity/its time.
3. Net efficiency = sum of all rates. Time to fill/empty = Capacity ÷ Net efficiency.

💡 Example: Pipe A fills a tank in 10 hrs, Pipe B empties it in 15 hrs. Capacity=LCM(10,15)=30. A=+3/hr, B=−2/hr. Net=+1/hr. Time to fill=30 hrs.

⚠️ Always assign a negative sign to outlet (draining) pipes — forgetting this sign is the most common error here.

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