Let D be a simple graph on 10 vertices such that there is a vertex of degree 1, a vertex of degree 2, a vertex of degree 3, a vertex of degree 4, a vertex of degree 5, a vertex of degree 6, a vertex of degree 7, a vertex of degree 8 and a vertex of degree 9. What can be the degree of the last vertex?

Correct Answer: 2
We know that sum of degrees of all vertices = 2X no of edges. Say number of edges is E. Degree of last vertex is x, 1+2+3+4+5+6+7++8+9+x = 2XE =>45+x = 2XE Now putting options we get answer 0 or 5 But one vertex of degree 9 means it connected to all other vertexes. So, the degree must be 5.