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 B is turned off after:

Two pipes A and B can fill a cistern in 37 1/2 minutes and 45 minutes

1.	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 B is turned off after:
A.	9 min.
B.	5 min.
C.	15 min.
D.	10 min.
Answer» B. 5 min.