Pipe A fills a tank in 6 minutes. Pipe B can fill the same tank 2 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?
Select: A) 0.5 B) 1.0 C) 1.5 D) 2.0 E) 2.5

Rationale: "a ' s rate is 1 / 6 and b ' s rate is 1 / 3 . the combined rate is 1 / 6 + 1 / 3 = 1 / 2 the pipes will fill the tank in 2 minutes . the answer is d ."
Answer: D