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 one:	A) 3	B) 4	C) 5	D) 6	E) 7
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 6.
<result>D</result>
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?