Pipes and Cisterns MCQs: Practice Questions and Answers

Pipes and cisterns questions are time-and-work questions in disguise: every pipe contributes a rate (tank per hour), and rates add — an emptying pipe simply contributes a negative rate. Below are 12 practice MCQs with worked solutions. Attempt each before reading the answer.

The One Method You Need

• Rate: a pipe filling a tank in n hours works at 1/n tank per hour.
• Net rate: add filling rates, subtract emptying rates. Time = work ÷ net rate.
• Pipe closed midway: compute the work done in the first phase, then divide the remaining work by the remaining pipe''s rate.

Practice Questions

Q1. Pipes A and B can fill a tank in 6 and 8 hours. Both are opened; after 2 hours B is closed. Total time to fill the tank?

(a) 4 h (b) 4 h 30 min (c) 5 h (d) 5 h 30 min

Answer: (b) 4 h 30 min. In 2 hours: 2 × (1/6 + 1/8) = 7/12. Remaining 5/12 by A alone: (5/12) ÷ (1/6) = 2.5 h. Total = 2 + 2.5 = 4.5 hours.

Q2. A pipe fills a tank in 8 hours; another empties it in 12 hours. Both open together — time to fill?

(a) 20 h (b) 22 h (c) 24 h (d) 28 h

Answer: (c) 24 h. Net rate = 1/8 − 1/12 = 1/24 tank per hour.

Q3. Pipes A and B fill a tank in 12 and 18 hours. Both open together — time to fill half the tank?

(a) 3 h 36 min (b) 4 h (c) 4 h 30 min (d) 7 h 12 min

Answer: (a) 3 h 36 min. Combined rate = 1/12 + 1/18 = 5/36. Half tank: (1/2) ÷ (5/36) = 3.6 h = 3 h 36 min.

Q4. Pipes A (8 h) and B (12 h) are opened together; after 2 hours A is closed. Total time to fill the cistern?

(a) 7 h (b) 8 h (c) 9 h (d) 10 h

Answer: (c) 9 h. In 2 hours: 2 × (1/8 + 1/12) = 5/12. Remaining 7/12 by B: (7/12) ÷ (1/12) = 7 h. Total = 9 hours.

Q5. A pipe fills in 6 hours; an emptying pipe drains in 8 hours. Both open — time to fill?

(a) 14 h (b) 18 h (c) 24 h (d) 48 h

Answer: (c) 24 h. Net rate = 1/6 − 1/8 = 1/24.

Q6. Pipe A fills in 10 hours, pipe B empties in 15 hours. Both open — time to fill half the tank?

(a) 10 h (b) 12 h (c) 15 h (d) 30 h

Answer: (c) 15 h. Net rate = 1/10 − 1/15 = 1/30; half tank = (1/2) ÷ (1/30) = 15 hours.

Q7. Pipes A (15 h) and B (20 h): A runs alone for 5 hours, is closed, and B finishes. Total time?

(a) 16 h 40 min (b) 18 h 20 min (c) 13 h 20 min (d) 20 h

Answer: (b) 18 h 20 min. A does 5/15 = 1/3. B fills the remaining 2/3 in (2/3) × 20 = 13 h 20 min. Total = 5 + 13⅓ = 18 h 20 min.

Q8. A pipe fills a tank in 8 hours. After half the tank is filled, an emptying pipe (16 h) is also opened. Total time to fill?

(a) 10 h (b) 12 h (c) 14 h (d) 16 h

Answer: (b) 12 h. First half: 4 h. Then net rate = 1/8 − 1/16 = 1/16, so the second half takes (1/2) ÷ (1/16) = 8 h. Total = 12 hours.

Q9. Pipes A (10 h) and B (12 h) are opened together; after 3 hours B is closed. Total time to fill?

(a) 6 h 30 min (b) 7 h (c) 7 h 30 min (d) 8 h

Answer: (c) 7 h 30 min. In 3 hours: 3 × (1/10 + 1/12) = 33/60 = 11/20. Remaining 9/20 by A: (9/20) ÷ (1/10) = 4.5 h. Total = 7.5 hours.

Q10. Pipes A (16 h) and B (24 h) are opened together. Time to fill half the tank?

(a) 4 h 48 min (b) 5 h 20 min (c) 9 h 36 min (d) 10 h

Answer: (a) 4 h 48 min. Combined rate = 1/16 + 1/24 = 5/48. Half tank: (1/2) ÷ (5/48) = 4.8 h = 4 h 48 min.

Q11. A pipe fills in 5 hours; an emptying pipe drains in 10 hours. Both open — time to fill?

(a) 8 h (b) 10 h (c) 12 h (d) 15 h

Answer: (b) 10 h. Net rate = 1/5 − 1/10 = 1/10.

Q12. Pipes A (12 h) and B (16 h): A runs alone for 2 hours, is closed, and B finishes. Total time?

(a) 13 h 20 min (b) 14 h (c) 15 h 20 min (d) 16 h

Answer: (c) 15 h 20 min. A does 2/12 = 1/6. B fills the remaining 5/6 in (5/6) × 16 = 13 h 20 min. Total = 2 + 13⅓ = 15 h 20 min.

Practice More

Attempt timed pipes-and-cisterns sets with per-question solutions in Pareeksha practice sets, or take a full-length quant mock in the mock test section.