need help...!!! There is a set with N element, the N element consist of k type of object. that is n1 of first type object, n2 second type object, ..., n(k-1) (k-1)th object and the remainder is zero. How many possible way to arrange the element of this set in a row such that the different between two row that we arrange is exactly d element, specially for d=2. "sorry for my english"