Pipes A and B can fill a tank in 20 min and 30 min respectively. If both are opened together, how long does it take to fill the tank?
Aptitude
Pipes and Cistern
Difficulty: Easy
Choose an option
Answer
Correct Answer: 12 minutes
Explanation
Problem restatementCombine two constant fill rates to get the time to fill one tank.
Given data
- A's rate = 1/20 tank per min
- B's rate = 1/30 tank per min
Concept/ApproachAdd rates for parallel work: total rate = 1/20 + 1/30 = 1/12 tank per min ⇒ time = reciprocal.
Step-by-step calculation1/20 + 1/30 = (3 + 2)/60 = 5/60 = 1/12Time = 1 / (1/12) = 12 minutes
Common pitfallsAveraging times instead of adding rates.
Final Answer12 minutes