Problem: a work as fast as b . if b can complete a work in 20 days independently , the number of days in which a and b can together finish the work in ?
Rationale: "ratio of rates of working of a and b = 2 : 1 ratio of times taken = 1 : 2 a ' s 1 day work = 1 / 10 b ' s 1 day work = 1 / 20 a + b 1 day work = 1 / 10 + 1 / 20 = 3 / 20 = > 20 / 3 = 6 2 / 3 a and b can finish the work in 6 2 / 3 days answer is e"
annotated_formula: inverse(add(inverse(20), multiply(const_2, inverse(20))))