The question is as follows:

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What is the maximum number of points of intersection between the diagonals of a convex octagon

(8-vertex planar polygon)? Note that a polygon is said to be convex if the line segment joining any two

points in its interior lies wholly in the interior of the polygon. Only points of intersection between diagonals

that lie in the interior of the octagon are to be considered for this problem.

The answer, given, is 70.

I cannot find the logic. Help me please!!