Problem: a cistern is filled by a tap in 7 1 / 2 hours . due to leak in the bottom of the cistern , it takes half an hour longer to fill the cistern . if the cistern is full how many hours will it take the leak to empty it ?
Rationale: "filling rate - leak rate = net rate 1 / 7.5 - leak rate = 1 / 8 leak rate = 2 / 15 - 1 / 8 = 1 / 120 the answer is c ."
annotated_formula: divide(1, subtract(divide(const_1, add(7, divide(1, 2))), divide(1, const_4)))