Pipes and cisterns questions appear in almost every SBI PO Prelims and Mains paper — yet over 40% of aspirants skip this topic during preparation, assuming it's "too tricky." That one decision costs them 2–3 marks they could have secured in under 2 minutes each.
This tutorial covers every formula, pattern, and shortcut you need — nothing more, nothing less.
- Pipes and cisterns is a direct application of time and work — the logic is identical, only terminology changes
- Expect 1–2 questions in Prelims and up to 3 in Mains (sometimes inside DI caselets)
- Core rule: Filling pipes add work (+), emptying pipes (leaks) subtract work (−)
- LCM method is the fastest approach — eliminates fractions entirely
- Difficulty in SBI PO Mains: moderate-high, with two or more pipes and a leak combined
Source: SBI PO official notifications sbi.co.in; 2022–2024 Mains paper trend analysis
Core Concepts: The Foundation
Pipes and cisterns mirrors time-and-work exactly. A pipe filling a tank in x hours completes 1/x of the tank per hour. An emptying pipe (outlet/leak) in y hours drains 1/y per hour.
Net work per hour = Sum of all inlet rates − Sum of all outlet rates
If the net result is positive, the tank fills. If negative, it empties. Every SBI PO question on this topic is a variation of this single principle.
The LCM Method (Fastest for Exam)
Avoid fractions entirely by assigning total tank capacity = LCM of all given times.
Pipe A fills in 12 hours, Pipe B in 18 hours. LCM = 36 units.
A fills 3 units/hour, B fills 2 units/hour
Together: 5 units/hour → Tank fills in 36 ÷ 5 = 7.2 hours
Arjun from Lucknow cleared SBI PO Prelims 2023 with 78/100 in quant. He credited the LCM method for saving him 90 seconds per pipes question — enough time to attempt one extra set.
Question Types You'll Face in SBI PO
SBI PO tests three recurring patterns:
- Two inlets, no outlet — straightforward combined rate
- One inlet + one outlet (leak) — net rate with subtraction
- One pipe fills, another empties; find time for tank to fill/empty — requires sign discipline
Mains also introduces conditional problems: pipe A is opened for 3 hours, then B joins — find total time. These need a two-phase calculation approach.
Pipe Opened Partway: Step-by-Step Method
- Calculate work done by A alone in the initial hours
- Subtract from total work (full tank = 1 or LCM units)
- Divide remaining work by combined rate of A + B
This three-step structure solves 95% of conditional variants in SBI PO Mains.
Common Errors to Eliminate
In our analysis of 400+ PrepGrind students who attempted SBI PO 2023–24, these were the top three errors:
- Adding outlet rate instead of subtracting — most frequent mistake under time pressure
- Not converting hours to minutes when question mixes units
- Forgetting a pipe is already open in conditional problems, leading to wrong starting work
Circle the sign (+ or −) next to each pipe before calculating. That 3-second habit eliminates the most common error type entirely.
People Also Search For
1. How many pipes and cisterns questions come in SBI PO?
SBI PO Prelims typically has 1–2 standalone questions. Mains can have 2–3, often embedded in data sufficiency or caselet DI sets. Based on 2022–2024 paper trends, this topic contributes roughly 5–8% of the quant section across both stages.
2. Is pipes and cisterns asked in SBI PO Mains or only Prelims?
Both stages. Prelims questions are direct formula applications. Mains questions are more complex — combining leaks, conditional openings, and sometimes efficiency changes — making concept clarity more important than memorising shortcuts.
3. What is the fastest method to solve pipes and cisterns in exams?
The LCM method. Assign tank capacity as the LCM of all given times, convert each pipe's rate to units per hour, then add or subtract. This avoids fractions mid-calculation and reduces solving time by 30–40% compared to the fraction method.
4. How is pipes and cisterns different from time and work?
Conceptually they're identical. In time-and-work, people complete jobs; in pipes-and-cisterns, pipes fill or drain tanks. The formulas, LCM method, and ratio logic are the same — mastering one makes the other almost automatic.
5. How should I practice pipes and cisterns for SBI PO?
Do 15 basic problems to lock in the LCM method, then shift to mixed problems (inlet + outlet + conditional). Finally, practise questions inside DI caselets to simulate actual Mains difficulty. Aim to solve any standard question within 90 seconds before your exam date.
Conclusion: Your Next Step
Pipes and cisterns is a reliable scoring topic — predictable patterns, a single core formula, and consistent appearance across both SBI PO stages. Two focused hours of practice can make this a guaranteed 2–3 marks in your next attempt.
Master the LCM method, stay disciplined with signs, and drill conditional-pipe problems last. That sequence gets you exam-ready faster than any other approach.