Problem: two pipes a and b can fill a tank in 36 hours and 45 hours respectively . if both the pipes are opened simultaneously , how much time will be taken to fill the tank ?
Rationale: "1 / ( 1 / 36 + 1 / 45 ) = 180 / 9 = 20 hrs answer : a"
annotated_formula: divide(const_1, add(divide(const_1, 36), divide(const_1, 45)))