Pipe A can fill a tank in 30 minutes. Pipe B can fill the same tank 6 times faster than pipe A. If both the pipes are connected to the tank so that they fill the tank simultaneously, how long will it take for the empty tank to overflow?	Choose the correct choice from the following choices:
A) 4 minutes B) 15 / 4 minutes C) 192 / 7 minutes D) 224 / 7 minutes E) 28 minutes

Rationale: "pipe a can fill a tank in 30 minutes - - > the rate of a = 1 / 30 tank / minute ; pipe b can fill the same tank 6 times fasterthan pipe a - - > the rate of b = 1 / 30 + 6 / 30 = 7 / 30 tank / minute . their combined rate = 1 / 30 + 7 / 30 = 4 / 15 tank / minute ; therefore , a and b can fill the tank in 15 / 4 minutes . answer : b"
Answer: B