Pipe A fills a tank in 56 minutes. Pipe B can fill the same tank 7 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? Choices:
A) 4	B) 5	C) 6	D) 7	E) 8

Rationale: a ' s rate is 1 / 56 and b ' s rate is 1 / 8 . the combined rate is 1 / 56 + 1 / 8 = 1 / 7 the pipes will fill the tank in 7 minutes . the answer is d .
Answer: D