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A cistern can be filled by two pipes A and B in 30 and 36 minutes respectively. When the cistern is empty, both the pipes are opened for a certain time, but being partially clogged, only 5/6 of the full quantity of water flows through the pipe A and only 9/10 through the pipe B. the obstructions, however, being suddenly removed, the cistern is filled in 15 1/12 minutes from that moment. How long was it before the full flow of water began?

A.	1 min	B.	2 min
C.	1 ½ min	D.	2 ½ min

Answer and Explanation

Answer:1 min

Explanation:
Let the pipes A and B be open for x minutes before the full flow began.
x (5 /(6×30) + 9 / (10×36)) + 31/2(1/30 + 1/36) = 1
x(1/36 + 1/40) + 31/2(1/30 + 1/36) =1
x(19/360) + 31/2(11/180) = 1
19x + 341 / 360 = 1
19x = 360 – 341 =19
x = 19/19
x = 1 min

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