A graph G has 21 Edges, 3 vertices of degree 4 and other vertices are of degree 3. Find the number of vertices in G.
Ans: It is specified that graph G has 21 edges, so total degree of graph is 42. It is as well given that three vertices are of degree 4 and other vertices are degree 3. Assume number of vertices of degree 3 is y. After that
3y + 4x3 = 42
⇒ y = (42 - 12) / 3 = 10.