pipe a fills a tank in 12 minutes . pipe b can fill the same tank 3 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 / 12 and b ' s rate is 1 / 4 . the combined rate is 1 / 12 + 1 / 4 = 1 / 3 the pipes will fill the tank in 3 minutes . the answer is d ."
inverse(add(divide(const_1, 12), divide(3, 12)))