Exam path · Pipes & Cisterns
Pipes and cisterns questions give the time each pipe takes to fill or empty a tank alone, then ask how long it takes when several run together. A filling pipe is a positive rate and an emptying pipe or leak is a negative one; add the rates and divide the tank's size by the total. It is time and work with one sign flipped, never with hours added directly.
Pipes and cisterns is the same rate-and-work idea as time and work, wearing a tank instead of a job, and it appears about as often — one or two marks in banking and SSC prelims. The only new step is the outlet pipe, whose rate is subtracted rather than added, and most wrong answers in this topic come from treating that subtraction as an addition.
Try it against the clock. Pipes & Cisterns runs at 45s easy, 55s medium and 70s hard. The clock is stamped and judged on our server, so the limit you see is the deadline that is actually enforced — and it starts when you tap Start, not while the question is loading.
If Pipe A takes 12 hours and Pipe B takes 24, call the tank 24 units. Every rate below becomes a whole number, so there is no fraction to carry through the question.
Pipe A adds 24 ÷ 12 = 2 units an hour, Pipe B adds 24 ÷ 24 = 1. An outlet or leak that empties the full tank in 8 hours removes 24 ÷ 8 = 3 units an hour — the same conversion, just a rate to subtract instead of add.
Net rate = (sum of inlet rates) − (sum of outlet rates). Time to fill = tank size ÷ net rate. If the net rate comes out negative, the tank never fills — the outlets are winning.
If one pipe runs alone for k hours before a second joins, subtract what it filled in those k hours from the tank first, then divide the remainder by the combined rate for the time after both are open.
| What the paper says | What it means |
|---|---|
| Pipe A: 12 h, Pipe B: 24 h, together | Rates 2 and 1 out of 24 units — together 3, so 8 hours |
| Two inlets plus one outlet | Add the inlet rates, subtract the outlet rate, then divide |
| A runs alone, then B joins | Subtract what A already filled, then divide the rest by the combined rate |
| A leak empties an already-full tank in x hours | Its rate is −(tank ÷ x), same units as every inlet |
Generated by the same engine that mints the ComputePrep daily. Each answer is computed from the numbers printed in the question, and the walkthrough below each one is the engine's own working — not a solution written afterwards. Reload this page's live drill and you get different numbers.
Pipe A fills a cistern in 10 hours. Pipe B fills it in 20 hours. Pipe C empties the full cistern in 40 hours.
All three pipes are opened together on an empty cistern.
In how many hours will the cistern be full?
Pipe A fills a cistern in 12 hours. Pipe B fills it in 20 hours. Pipe C empties the full cistern in 30 hours.
All three pipes are opened together on an empty cistern.
In how many hours will the cistern be full?
Pipe A fills a cistern in 8 hours. Pipe B fills it in 12 hours. Pipe C empties the full cistern in 24 hours.
All three pipes are opened together on an empty cistern.
In how many hours will the cistern be full?
The wrong options are not random numbers. Each one is the result of a specific careless error for this topic, so picking one tells you which habit is costing you marks.
If Pipe A takes 12 hours and Pipe B takes 24, together they do not take 36 hours or 18. Hours are not additive; rates are, and the together-time is always faster than the quicker pipe alone.
An outlet subtracts from the net rate. Adding all the given times' rates together, outlet included, gives a plausible number that fills the tank too fast — and it is always on the options list.
When one pipe runs alone before a second opens, the remaining work is the tank minus what the first pipe already filled — not the whole tank divided by the combined rate. Skipping that subtraction overstates the time needed.
The specific wrong numbers this topic produces, and what each one tells you about the step you took. If you have just got a question wrong and want to know which habit did it, start here.
18 hours is the average of 12 and 24, but times do not average, rates do. Taking the tank as 24 units, A adds 2 an hour and B adds 1, so together they add 3 an hour and fill it in 24 ÷ 3 = 8 hours — well under either pipe alone, which any correct together-time must be.
Because C is an outlet, its rate is subtracted, not added. On a 36-unit tank, A and B add 3 and 2 units an hour and C removes 1, for a net of 4, giving 36 ÷ 4 = 9 hours. Adding all three rates as if C also filled gives a faster, wrong time.
Because Pipe A already filled part of the tank before B joined. On a 36-unit tank with A at 3 units an hour, 2 hours alone fills 6 units, leaving 30. Both pipes together add 5 units an hour, so the remaining 30 units take 6 more hours — the head start has to be subtracted first.
Yes. They are a standard arithmetic topic in IBPS, SBI, SSC and railway quantitative aptitude sections, usually worth one or two marks and closely related to time and work — the same rate method solves both.
An inlet's rate is added to the total; an outlet or leak's rate is subtracted. Everything else about the method — take the tank as the LCM of the given hours, convert to rates, combine, divide — stays the same.
About 45 to 70 seconds in prelims, which is what this drill enforces on the server: 45 seconds on easy, 55 on medium and 70 on hard.
Mathematically, yes. A filling pipe is a positive rate and an emptying pipe is a negative one, so the same add-the-rates method covers both — the tank just replaces the job.
Yes, free with no account. Every question is generated fresh, so the numbers are never the same twice.
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