a cistern which could be filled in 4 hours takes one hour more to be filled owing to a leak in its bottom . if the cistern is full in what time will the leak empty it ?
"1 / 4 - 1 / x = 1 / 10 = > 20 / 3 hrs answer : d"
inverse(subtract(divide(const_1, 4), divide(const_1, const_10)))