Problem: a man can do a piece of work in 5 days , but with the help of his son , he can finish it in 3 days . in what time can the son do it alone ?
Rationale: "son ' s 1 day work = 1 / 3 - 1 / 5 = 2 / 15 son alone can do the work in 15 / 2 days = 7 1 / 2 days answer is c"
annotated_formula: divide(multiply(5, 3), subtract(5, 3))