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?  Options: A) 4 B) 5 C) 6 D) 7 E) 8
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 7.
<result>D</result>
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?