How many k-vertex paths does the graph Kn have? why? How many k-vertex paths does the graph Kn1,n2 have?why?

If G = (V, E) is a graph with |V| = n, how many induced subgraphs does G have?why? How many spanning subgraphs of Kn1,n2 have exactly m edges?why? Let G = (V, E) be a graph with m edges. How many spanning subgraphs of G have exactly k edges?why?