if a fills a tank in 30 mins , and b fills the same tank in 20 mins , and c emptys it in 40 min , if a , b , c works together how much time will the tank take to fill ?
1 / 30 + 1 / 20 - 1 / 40 = 7 / 120 so total time = 120 / 7 = 17.14 mins answer : c
divide(const_1, subtract(add(divide(const_1, 30), divide(const_1, 20)), divide(const_1, 40)))