number of quadilaterals that can be made using vertices of a polygon of 10 sides as their vertices and having exactly 2 sides common with polygon.

Hint: Firstly select two sides out of 10 sides of polygon i.e. 10C2 then after selecting this two sides our 3 + 2 vertices are gone, now we are left with 5 more vertices, Out of these 5 select any two vertices to complete the quadilaterals i.e. 5C2. So 10C2*5C2

Thanks, Vijay