a cistern is normally filled in 4 hours but takes two hours longer to fill because of a leak in its bottom . if the cistern is full , the leak will empty it in ?
"1 / 4 - 1 / x = 1 / 6 x = 12 answer : e"
inverse(subtract(divide(const_1, 4), divide(const_1, add(4, const_2))))