Pipes A, B and C can fill an empty tank $$\frac{30}{7}$$ hours, if all the three pipes are opened simultaneously. A andB are filling pipes and C is an emptying pipe. Pipe A can fill the tank in 15 hours and pipe C can empty it in 12 hours. In how long (in hours) can pipe B alone fill the empty tank?

A

3

B

5

C

6

D

4