Two pipes A and B can fill a cistern in 37 1/2 minutes and 45 minutes respectively. Both pipes are opened. The cistern will be filled in just half an hour, if the pipe B is turned off after? Choose the correct answer:
A) 7
B) 6
C) 9
D) 7
E) 2
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.   x(2/75 + 1/45) + (30- x) 2/75 = 1   11x + 180 - 6x = 225 =&gt; x = 9   Answer: 6
<result>B</result>
Two pipes A and B can fill a cistern in 37 1/2 minutes and 45 minutes respectively. Both pipes are opened. The cistern will be filled in just half an hour, if the pipe B is turned off after?