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

Rationale: "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 c ."
Answer: C