Pipe A fills a tank in 42 minutes. Pipe B can fill the same tank 6 times as fast as pipe A. If both the pipes are kept open when the tank is empty, how many minutes will it take to fill the tank?  Choose the correct choice from the following choices: A) 3 B) 4 C) 5 D) 6 E) 7

Rationale: "a ' s rate is 1 / 42 and b ' s rate is 1 / 7 . the combined rate is 1 / 42 + 1 / 7 = 1 / 6 the pipes will fill the tank in 6 minutes . the answer is d ."
Answer: D