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2019 AMC 8 Question 6

 
 
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2019 AMC 8 Question 6
by Ellen Zong - Thursday, April 9, 2020, 12:57 AM
 

There are 81 grid points (uniformly spaced). Point P is in the center of the square. Given that Point Q is randomly chosen among the other 80 points, what is the probability that the line PQ is the line of symmetry for the square? 

 
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Re: 2019 AMC 8 Question 6
by John Lensmire - Thursday, April 9, 2020, 12:25 PM
 

A square has different lines of symmetry. For example, a vertical line through $P$ is one such line of symmetry (because then the square is the same to the left and to the right of the line):


Note this line contains $P$ and $4+4 = 8$ other points. Therefore, if $Q$ is one of these $8$ points, $PQ$ is a line of symmetry for the square.

With one line of symmetry done, how many other lines of symmetry do we need to consider? How many other points do they contain?