Corollary4.6LetS={o·a1,o·a2,…,oak},and rk. Then the number of r-combinations of' s so that each of the k types of objects occurs at least once equals c(r-1, k-1). Proofe r-combinations of s so that each of the k types of objects occurs at least once, aj,a2,. ak, r-k combinations r-k combinations, +a1,a2,-. ak, r-combinations of s so that each of the k types of objects occurs at least once▪ Corollary 4.6 Let S={·a1 ,·a2 ,…,·ak },and rk . Then the number of r-combinations of S so that each of the k types of objects occurs at least once equals C(r-1,k-1). ▪ Proof: r-combinations of S so that each of the k types of objects occurs at least once, a1 ,a2 ,…,ak， ▪ r-k combinations ▪ r-k combinations, +a1 ,a2 ,…,ak，r-combinations of S so that each of the k types of objects occurs at least once