Three taps P Q and R can fill a tank in 12 15 and 20 hours respectively If P is open all the time an...........

Quant (SSC) Pipes&cistern

Three taps P, Q and R can fill a tank in 12, 15 and 20 hours respectively. If P is open all the time and Q, R are open for one hour each alternatively, the tank will be full in

3 hours

2 hours

7 hours

4 hours

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Explanation:

(P + Q)’s 1 hour work = 1/12 + 1/15 = 3/20
(P + R)’s 1 hour work = 1/12 + 1/20 = 2/15
For 2 hrs = (3/20 + 2/15) = 17/60
For 6 hrs = (3*17/60) = 17/20
Remaining Part = 1 – 17/20 = 3/20 filled by P and Q in 1 hour
Hence, option 3 is the correct answer.