GRE Permutation and geometry question 
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*if you join all the vertices of a heptagon, how many quadrilaterals will you get?

*How many heptagons can be drawn by joining the vertices of a polygon with 10 sides?

*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.

*

*You need to choose 4 vertices from 7 vertices of Heptagon. How do you do that?

*Same as 1, you need to select 7 out of 10.

*No of distance diagonals is n(n-3)/2; you can do this if you can get above 2.

