Problem: a man can do a job in 15 days . his father takes 20 days and his son finishes it in 15 days . how long will they take to complete the job if they all work together ?
Rationale: "1 day work of the three persons = ( 1 / 15 + 1 / 20 + 1 / 15 ) = 11 / 60 so , all three together will complete the work in 300 / 47 = 5.5 days . answer : c"
annotated_formula: divide(const_1, add(divide(const_1, 15), add(divide(const_1, 15), divide(const_1, 20))))