SSC Pipes and Cisterns Questions with Solutions and Tricks

Pipes and Cisterns is an application of Time and Work. Here, filling a tank is positive work and emptying is negative work. One question from this topic is common in the SSC CGL, CHSL and MTS papers, and the LCM method solves most of them in half a minute. This practice paper gives you 30 pipes and cisterns questions with answers and short solutions.

Practice Paper at a Glance

What Are Pipes and Cisterns? Basic Concept in Simple Words

A cistern (tank) is filled by inlet pipes and emptied by outlet pipes (or a leak). The questions ask how much time is needed to fill or empty the tank when different pipes work together.

• An inlet pipe adds water, so it works with a positive sign.
• An outlet pipe or leak removes water, so it works with a negative sign.
• Part of the tank filled in 1 hour by a pipe = 1 / (hours it takes to fill the whole tank).

Formulas and Shortcut Tricks for SSC Pipes and Cisterns

• LCM method: take the LCM of the times as the capacity of the tank. Divide it by each time to get the rate (units per hour). Add the inlet rates, subtract the outlet rates and divide the capacity by the net rate.
• Two inlet pipes taking a and b hours: time together = ab / (a + b).
• An inlet taking a hours and an outlet taking b hours (b > a): time to fill = ab / (b - a).
• Leak: if a pipe fills a tank in a hours and with a leak in b hours (b > a), the leak empties the full tank in ab / (b - a) hours.
• Closing a pipe midway: find the work done in the first part, subtract it from the capacity and find the time for the remaining part.
• Alternate pipes: find the work done in one cycle, count the full cycles and handle the remaining part.
• Identical pipes: pipes x time is constant. 6 pipes x 12 hours = 9 pipes x 8 hours.
• Volume to litres: 1 cubic metre = 1,000 litres.

Set 1: Basic Pipes and Cisterns Questions (Q1–Q10)

Q1. A pipe can fill a tank in 10 hours. What part of the tank does it fill in 1 hour?
Answer: 1/10
Solution: Part filled in 1 hour = 1 / 10.

Q2. Pipe A can fill a tank in 6 hours and pipe B in 12 hours. How long will they take together?
Answer: 4 hours
Solution: LCM = 12. A = 2 and B = 1 per hour. Together 3 per hour, so 12 / 3 = 4 hours.

Q3. An inlet pipe can fill a tank in 8 hours and an outlet pipe can empty it in 12 hours. If both are opened together, how long will the empty tank take to fill?
Answer: 24 hours
Trick: LCM = 24. Inlet = +3 and outlet = -2. Net = 1 per hour, so 24 hours.

Q4. Pipe A can fill a tank in 15 minutes and pipe B in 10 minutes. How long will they take together?
Answer: 6 minutes
Solution: Time = (15 x 10) / (15 + 10) = 150 / 25 = 6 minutes.

Q5. Pipe A can fill a tank in 20 minutes and pipe B can empty it in 30 minutes. If both are opened, in how much time will the tank be filled?
Answer: 60 minutes
Solution: Time = (20 x 30) / (30 - 20) = 60 minutes.

Q6. A pipe can fill a tank in 5 hours, but because of a leak it takes 6 hours. In how many hours will the leak alone empty the full tank?
Answer: 30 hours
Trick: Leak = (5 x 6) / (6 - 5) = 30 hours.

Q7. Three pipes can fill a tank in 10, 12 and 15 hours separately. How long will they take together?
Answer: 4 hours
Solution: LCM = 60. Rates = 6, 5 and 4. Together 15 per hour, so 60 / 15 = 4 hours.

Q8. A tank of capacity 1,200 litres is filled by a pipe at 40 litres per minute. How long will it take?
Answer: 30 minutes
Solution: Time = 1,200 / 40 = 30 minutes.

Q9. A pipe fills water at 12 litres per minute. How long will it take to fill a 360 litre tank?
Answer: 30 minutes
Solution: Time = 360 / 12 = 30 minutes.

Q10. Two pipes can fill a tank in 12 minutes and 16 minutes. How long will they take together?
Answer: 6 6/7 minutes (48/7)
Solution: Time = (12 x 16) / 28 = 192 / 28 = 48/7 = 6 6/7 minutes.

Set 2: Inlet, Outlet and Closing a Pipe (Q11–Q20)

Q11. Pipes A and B can fill a tank in 12 and 15 hours and pipe C can empty it in 20 hours. If all three are opened together, how long will the tank take to fill?
Answer: 10 hours
Solution: LCM = 60. A = 5, B = 4 and C = -3. Net = 6 per hour, so 60 / 6 = 10 hours.

Q12. Two pipes can fill a tank in 20 and 30 minutes and a third pipe can empty it in 40 minutes. If all are opened together, how long will it take to fill the tank?
Answer: 17 1/7 minutes (120/7)
Solution: LCM = 120. Rates = 6, 4 and -3. Net = 7 per minute, so 120 / 7 = 17 1/7 minutes.

Q13. Pipes A and B together can fill a tank in 8 hours. Pipe B alone can fill it in 12 hours. In how many hours can pipe A alone fill it?
Answer: 24 hours
Solution: LCM = 24. Together = 3 and B = 2, so A = 1 per hour. A takes 24 hours.

Q14. Pipe A can fill a tank in 6 hours and pipe B in 8 hours. Both are opened together, and after 2 hours pipe A is closed. How much more time will pipe B take to fill the rest of the tank?
Answer: 3 1/3 hours
Solution: LCM = 24. A = 4, B = 3. In 2 hours: 14 units. Remaining 10 units at 3 per hour = 3 1/3 hours.

Q15. A pipe can fill a tank in 4 hours. Due to a leak at the bottom, it takes 5 hours. How long will the leak take to empty the full tank?
Answer: 20 hours
Trick: Leak = (4 x 5) / (5 - 4) = 20 hours.

Q16. Pipes A and B can fill a tank in 30 and 45 minutes. Both are opened together, and after 9 minutes B is closed. In how much total time is the tank filled?
Answer: 24 minutes
Solution: LCM = 90. A = 3 and B = 2. In 9 minutes: 45 units. Remaining 45 units at 3 per minute = 15 minutes. Total = 24 minutes.

Q17. Two pipes can fill a tank in 10 and 15 minutes. A third pipe is opened, and then all three together fill the tank in 30 minutes. In how many minutes can the third pipe alone empty the full tank?
Answer: 7.5 minutes
Solution: 1/10 + 1/15 - 1/x = 1/30. So 1/x = 1/6 - 1/30 = 4/30 and x = 7.5 minutes.

Q18. A full tank is emptied by an outlet in 8 hours. How long does it take to empty half of the tank?
Answer: 4 hours
Solution: Half the tank takes half the time = 4 hours.

Q19. A pipe fills water at 25 litres per minute, and a leak empties 5 litres per minute. How long will it take to fill a 400 litre tank?
Answer: 20 minutes
Solution: Net rate = 20 litres per minute. Time = 400 / 20 = 20 minutes.

Q20. Pipes A and B can fill a tank in 10 hours, B and C in 15 hours and A and C in 12 hours. In how many hours will half the tank be filled if all three are opened?
Answer: 4 hours
Solution: 1/10 + 1/15 + 1/12 = 15/60 = 1/4, which is 2(A+B+C). So A+B+C = 1/8 per hour and the full tank takes 8 hours. Half the tank takes 4 hours.

Set 3: Mixed and Exam-Level Questions (Q21–Q30)

Q21. Pipes A and B can fill a tank in 5 and 10 hours and pipe C can empty it in 20 hours. If all three are opened together, in how many hours will the tank be filled?
Answer: 4 hours
Solution: LCM = 20. A = 4, B = 2 and C = -1. Net = 5 per hour, so 20 / 5 = 4 hours.

Q22. Pipes A and B can fill a tank in 40 and 60 minutes. Both are opened together, and after 10 minutes A is closed. How many more minutes will B take to fill the tank?
Answer: 35 minutes
Solution: LCM = 120. A = 3 and B = 2. In 10 minutes: 50 units. Remaining 70 units at 2 per minute = 35 minutes.

Q23. Tap A is 3 times as fast as tap B. Together they can fill a tank in 12 minutes. In how many minutes can A alone fill it?
Answer: 16 minutes
Solution: Let B = x. A = 3x. 4x = 1/12, so x = 1/48 and A = 3/48 = 1/16. A takes 16 minutes.

Q24. 6 identical pipes can fill a tank in 12 hours. How long will 9 such pipes take?
Answer: 8 hours
Trick: Pipes x hours = 6 x 12 = 72. Time for 9 pipes = 72 / 9 = 8 hours.

Q25. A pipe can fill a tank in 6 hours and another can empty it in 8 hours. Both are opened, and after 2 hours the outlet pipe is closed. How long in total does the tank take to fill?
Answer: 7.5 hours
Solution: Net rate for 2 hours = 1/6 - 1/8 = 1/24 per hour, so 1/12 is filled. Remaining 11/12 at 1/6 per hour = 5.5 hours. Total = 7.5 hours.

Q26. Two inlet pipes can fill a tank in 6 and 8 hours and an outlet can empty it in 12 hours. If all three are opened, how long will it take to fill 3/4 of the tank?
Answer: 3.6 hours
Solution: LCM = 24. Net = 4 + 3 - 2 = 5 per hour. Full tank = 24/5 = 4.8 hours. 3/4 of the tank = 3.6 hours.

Q27. Pipes A and B can fill a tank in 15 and 20 hours. They are opened alternately for 1 hour each, starting with A. In how many hours will the tank be filled?
Answer: 17 hours
Solution: LCM = 60. A = 4 and B = 3. One cycle (2 hours) fills 7 units. 8 cycles = 16 hours = 56 units. The remaining 4 units are filled by A in the 17th hour. Total = 17 hours.

Q28. A cistern measures 4 m x 3 m x 2 m. A pipe fills it at 100 litres per minute. How long will it take?
Answer: 4 hours
Solution: Volume = 24 cubic m = 24,000 litres. Time = 24,000 / 100 = 240 minutes = 4 hours.

Q29. One pipe is twice as fast as another and together they fill a tank in 12 minutes. How long will the slower pipe alone take?
Answer: 36 minutes
Solution: Let slower = x. Faster = 2x. 3x = 1/12, so x = 1/36. The slower pipe takes 36 minutes.

Q30. Pipes A and B can fill a tank in 30 and 20 minutes and pipe C can empty it in 15 minutes. If all are opened together, how long will it take to fill the tank?
Answer: 60 minutes
Solution: LCM = 60. A = 2, B = 3 and C = -4. Net = 1 per minute, so 60 minutes.

Common Mistakes in Pipes and Cisterns

• Treating an outlet pipe as an inlet pipe (forgetting the negative sign).
• Adding the times instead of adding the rates.
• Not subtracting the part filled when a pipe is closed midway.
• Using the wrong formula for a leak (it should be ab / (b - a)).
• Not converting litres and cubic metres correctly.
• Mixing minutes and hours in the same equation.

How to Practice These 30 Questions

1. Revise the time and work LCM method before starting.
2. Write the rates next to each pipe and mark inlet with + and outlet with -.
3. Solve Set 1 in 6 minutes, Set 2 in 12 minutes and Set 3 in 12 minutes.
4. Check the answer by calculating the part filled by each pipe.
5. Revisit your mistakes after one day.

Related Topics to Study Next

• Time and Work Questions
• Time, Speed and Distance Questions
• Boats and Streams Questions
• SSC Previous Year Papers

Frequently Asked Questions

What are pipes and cisterns?

Pipes and cisterns questions deal with filling and emptying a tank by inlet and outlet pipes. They are a type of time and work problem.

What is the formula for two pipes filling a tank?

If two pipes can fill a tank in a and b hours, together they take ab / (a + b) hours. For an inlet and an outlet, the time is ab / (b - a) when the outlet is slower than the inlet.

What is an inlet pipe and an outlet pipe?

An inlet pipe fills the tank and an outlet pipe (or a leak) empties it. In calculations, inlet is taken as positive work and outlet as negative work.

How do I solve leak questions?

If a pipe fills a tank in a hours and with a leak in b hours, the leak alone empties the full tank in ab / (b - a) hours.

How do I solve questions when a pipe is closed midway?

Find the part of the tank filled in the first period, subtract it from the full tank and divide the remaining part by the rate of the pipes still working.

Is the LCM method useful in pipes and cisterns?

Yes. Taking the LCM of the times as the capacity of the tank converts all rates into whole numbers and makes the addition and subtraction easy.

How many pipes and cisterns questions come in SSC exams?

Generally one question comes in a shift, and many times it is combined with time and work.

What is the relation between pipes and cisterns and time and work?

Both use the same idea: the work done per hour. A pipe filling a tank is like a worker doing work, and an outlet is like a worker undoing the work.

How do I solve alternate pipes questions?

Find the part filled in one cycle, divide to count full cycles, and then find the time for the remaining part with the pipe whose turn it is.

How should I prepare pipes and cisterns for SSC exams?

Learn the LCM method and formulas, solve one set a day, practise leak and alternate-pipe questions and revise time and work regularly.