Exams › IBPS PO › Quantitative Aptitude
A can fill a tank in 20 hours, while B can empty the same tank in 15 hours. Both pipes are employed on a half-filled tank, and after 10 hours A is replaced by a pipe whose efficiency is double that of A. Find the total time required to fill the tank.
- 20
- 25
- 30
- 32
Correct answer: 30
Solution
A fills at 1/20 tank per hour and B empties at 1/15 tank per hour, so together the net rate is 1/20 - 1/15 = -1/60 tank per hour. Starting from half-filled, in 10 hours the tank becomes 1/2 - 10/60 = 1/3 full. The new pipe fills at 2/20 = 1/10 tank per hour, so with B the net rate is 1/10 - 1/15 = 1/30 tank per hour; filling the remaining 2/3 takes 20 hours, so total time = 10 + 20 = 30 hours.
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