Two pipes A and B can fill a cistern in 37 ½ min and 45 minutes respectively. Both the pipes are opened. The cistern will be filled in just half an hour, if the pipe B is turned off after  Choices:
A) 6 minutes
B) 9 minutes
C) 45 minutes
D) 3 minutes
E) 4 minutes

Rationale: if pipe b is turned off after x mins , then ( 2 * 30 ) / 75 + x / 45 = 1 x / 45 = 1 - 60 / 75 = 1 / 5 x = 45 / 5 = 9 mins answer : b
Answer: B