There are $15$ points in a plane. Let's name three of them as $A,B,C$. It was observed that three points were colinear with $A$ and $B$, four points were colinear with $B$ and $C$, five points were colinear with $C$ and $A$. How many triangles can be made with these $15$ points with their values being positive?
Source: MAA
Combinatorics
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