A pipe can empty a tank in 60 minutes alone Another pipe whose diameter is twice the diameter of fir...........

Quant (SSC) Pipes&cistern

A pipe can empty a tank in 60 minutes alone. Another pipe whose diameter is twice the diameter of first pipe is also opened. Now find the time in which both pipe will empty the tank together.

8 min

10 min

12 min

14 min

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Explanation:

Time taken by pipe to empty the tank is inversely proportional to cross- sectional area.
So, time taken by second pipe will be = 60/4 = 15 min (πr2 = 1/60 and for second pipe 4πr2 = 1/T so we get T = 15 min)
Time taken by both to empty the pipe = (60*15)/75 = 12