Source: Official Guide Revised GRE 2nd Ed. Part 9; Section 6; #25


Eight points are equally spaced on a circle

Eight points are equally spaced on a circle. If 4 of the 8 points are to be chosen at random, what is the probability that a quadrilateral having the 4 points chosen as vertices will be a square? 1/70, 1/35, 1,7, 1/4, 1/2

2 Explanations


Eric Kuzmenko

You can also do this: 1×(3÷7)×(2÷6)×(1÷5)

probability of choosing the first vertex is 1
probability of choosing one of corresponding vertices of the square is then 3/7
probability of choosing another (3rd) vertex is 2/6
probability of choosing the last (4th) vertex is 1/5

You don't multiply this by two (although there are two different squares you can make) because the first vertex you've picked as immediately limited you to one of the squares.

Aug 12, 2014 • Comment


Gravatar Chris Lele, Magoosh Tutor

Dec 8, 2012 • Comment

Add Your Explanation

You must have a Magoosh account in order to leave an explanation.

Learn More About Magoosh