Quantitative Aptitude

Time and Work questions with solutions

Time and work (including pipes and cisterns) appears in every exam stream. The LCM method turns every question into simple whole-number arithmetic.

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Prepared by the CBTAdda content team · Updated 28 October 2026 · How we check our content

How to solve time and work questions

Take the total work as the LCM of the given days. If A takes 12 days and B 18, total work = 36 units; A does 3 units a day, B does 2.
Add rates when people work together and subtract when one undoes the work (an outlet pipe).
Days = total units ÷ combined units per day.

Common trap: Never add the days. A in 12 days and B in 18 days together take 7.2 days, not 30 or 15.

12 practice questions

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A and B together finish a work in 12 days. A alone can do it in 20 days. How long will B take alone?
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Answer: (B) 30 days. B's one-day work = 1/12 − 1/20 = 1/30, so B needs 30 days.
Pipe A fills a tank in 12 hours and pipe B empties it in 20 hours. If both are opened together, how long will it take to fill the empty tank?
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Answer: (C) 30 hours. Net filling per hour = 1/12 − 1/20 = 1/30, so the tank fills in 30 hours.
Pipe A fills a tank in 20 hours and pipe B empties it in 30 hours. If both are opened together, how long will it take to fill the empty tank?
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Answer: (D) 60 hours. Net filling per hour = 1/20 − 1/30 = 1/60, so the tank fills in 60 hours.
A can finish a work in 10 days and B in 15 days. In how many days can they finish it together?
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Answer: (C) 6 days. Together they do 1/10 + 1/15 = 25/150 of the work per day, so they need 150/25 = 6 days.
A and B together finish a work in 8 days. A alone can do it in 12 days. How long will B take alone?
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Answer: (B) 24 days. B's one-day work = 1/8 − 1/12 = 1/24, so B needs 24 days.
Pipe A fills a tank in 6 hours and pipe B empties it in 9 hours. If both are opened together, how long will it take to fill the empty tank?
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Answer: (C) 18 hours. Net filling per hour = 1/6 − 1/9 = 1/18, so the tank fills in 18 hours.
A and B together finish a work in 4 days. A alone can do it in 6 days. How long will B take alone?
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Answer: (C) 12 days. B's one-day work = 1/4 − 1/6 = 1/12, so B needs 12 days.
Pipe A fills a tank in 10 hours and pipe B empties it in 15 hours. If both are opened together, how long will it take to fill the empty tank?
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Answer: (B) 30 hours. Net filling per hour = 1/10 − 1/15 = 1/30, so the tank fills in 30 hours.
Pipe A fills a tank in 8 hours and pipe B empties it in 12 hours. If both are opened together, how long will it take to fill the empty tank?
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Answer: (D) 24 hours. Net filling per hour = 1/8 − 1/12 = 1/24, so the tank fills in 24 hours.
A, B and C can finish a work alone in 12, 15 and 20 days respectively. In how many days will they finish it working together?
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Answer: (C) 5 days. Together: 1/12 + 1/15 + 1/20 = 1/5 of the work per day, so they need 5 days.
A can finish a work in 20 days and B in 30 days. In how many days can they finish it together?
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Answer: (A) 12 days. Together they do 1/20 + 1/30 = 50/600 of the work per day, so they need 600/50 = 12 days.
A and B together finish a work in 6 days. A alone can do it in 10 days. How long will B take alone?
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Answer: (B) 15 days. B's one-day work = 1/6 − 1/10 = 1/15, so B needs 15 days.