pipe a fills a tank in 6 minutes . pipe b can fill the same tank 2 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 ?
"a ' s rate is 1 / 6 and b ' s rate is 1 / 3 . the combined rate is 1 / 6 + 1 / 3 = 1 / 2 the pipes will fill the tank in 2 minutes . the answer is d ."
inverse(add(divide(const_1, 6), divide(2, 6)))