To fill a certain tank, pipes A, B and C take 20 minutes, 15 minutes and 12 minutes respectively. If the three pipes are opened every alternate minute, how long will it take to fill the tank?
Part filled by 3 pipes in 3 minutes = $$\frac{1}{20} + \frac{1}{15} + \frac{1}{12} = \frac{3 + 4 + 5}{60} = \frac{1}{5}$$
1/5th of the tank is filled in 3 minutes then whole tank is filled in 5 x 3 = 15min
Hence, option D is the correct answer.
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