a cistern is filled by a tap in 6 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 ?
"filling rate - leak rate = net rate 1 / 6.5 - leak rate = 1 / 7 leak rate = 2 / 13 - 1 / 7 = 1 / 91 the answer is d ."
divide(1, subtract(divide(1, add(6, divide(1, 2))), divide(1, const_4)))