Exams › SSC CGL (Prelims) › General

Two pipes A and B can fill a tank in 10 and 15 minutes respectively. After both are opened together for 3 minutes, the rate of pipe A becomes one-third, and pipe B becomes 2.5 times its original rate. In how many more minutes will the tank be full?

1. 2 min 15 sec
2. 3 min 10 sec
3. 2 min 30 sec
4. 1 min 45 sec

Correct answer: 2 min 30 sec

Solution

Initially, A fills \(1/10\) tank/min and B fills \(1/15\) tank/min, so together they fill \(1/6\) tank/min. In 3 minutes, they fill \(1/2\) tank. After that, A’s rate becomes \(\frac{1}{3}\cdot\frac{1}{10}=\frac{1}{30}\) and B’s rate becomes \(2.5\cdot\frac{1}{15}=\frac{1}{6}\), so combined rate is \(\frac{1}{5}\) tank/min. Remaining half tank takes \(\frac{1/2}{1/5}=2.5\) minutes = 2 min 30 sec.

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