Problem: a can do a piece of work in 10 days and b alone can do it in 20 days . how much time will both take to finish the work ?
Rationale: "this question can be solved by different methods . we need to conserve time in exams so solving this problem using equations is the good idea . time taken to finish the job = xy / ( x + y ) = 10 x 20 / ( 10 + 20 ) = 200 / 30 = 6.666 days answer : c"
annotated_formula: divide(const_1, add(divide(const_1, 10), divide(const_1, 20)))