Subsets of fixed size n(m-1)…(m-k+1) n! k(k-1)·1 k!(n-k)！ of ordered k-subsets:n(n-1)...(n-+1) of permutations of a k-set:(-1)...1 Subsets of fixed size ￾n k ⇥ = n(n ￾ 1)···(n ￾ k + 1) k(k ￾ 1)··· 1 = n! k!(n ￾ k)! # of ordered k-subsets: n(n ￾ 1)···(n ￾ k + 1) # of permutations of a k-set: k(k ￾ 1)··· 1