GRE Permutation and geometry question

Question

1. if you join all the vertices of a heptagon, how many quadrilaterals will you get?
2. How many heptagons can be drawn by joining the vertices of a polygon with 10 sides?
3. A polygon has 20 diagonals,how many sides does it have ? Are there are formula showing relation between diagonal and no of sides of a polygon?

These two questions are similar. Any clues to help me solve the problem.

Answer 1

1. You need to choose 4 vertices from 7 vertices of Heptagon. How do you do that?
2. Same as 1, you need to select 7 out of 10.
3. No of distance diagonals is n(n-3)/2; you can do this if you can get above 2.