Name: nick
Who is asking: Student
Level: Middle

Question:
What is the probability that four different points chosen at random from the fifteen equally-spaced points shown are the vertices of a square? I need to have my answer expressed as a common fraction. Help! I don't know how to write it.

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Hi Nick

First notice that there are 11 possible squares: 8 1by1's and 3 2by2's. I don't know what you've been taught about combinations but you also need to know the total number of ways to pick 4 points from 15 points; call this number N, your answer should be 11/N. N is quite large; well over 1000.