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Two pipes A and B can fill a cistern in 37 \(\frac{1}{2}\) minutes and 45 minutes respectively. Both pipes are opened. The cistern will be filled in just half an hour, if the B is turned off after:

Options:
A .  5 min.
B .  9 min
C .  10 min.
D .  15 min.
Answer: Option B

Let B be turned off after x minutes. Then,


Part filled by (A + B) in x min. + Part filled by A in (30 -x) min. = 1.


So, \(x\left(\frac{2}{75}+\frac{1}{45}\right)+(30-x).\frac{2}{75}=1\)


\(\frac{11x}{225}+\frac{(60-2x)}{75}=1\)


11x + 180 - 6x = 225.


 x = 9.


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