Pipe A fills a tank in 20 minutes. Pipe B can fill the same tank 4 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) 2 B) 3 C) 4 D) 5 E) 6
A's rate is 1/20 and B's rate is 1/5.   The combined rate is 1/20 + 1/5 = 1/4   The pipes will fill the tank in 4 minutes.   The answer is 4.
<result>C</result>
Pipe A fills a tank in 20 minutes. Pipe B can fill the same tank 4 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?