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?

A. 5 min
B. 9 min
C. 10 min
D. 15 min
Explanation:
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 => x = 9