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Squares Having a Common Side


Date: 03/08/99 at 08:57:27
From: suresh kumar
Subject: Probability

In choosing two squares from a chess board, what is the probability 
that they have a common side?


Date: 03/08/99 at 16:25:11
From: Doctor Anthony
Subject: Re: Probability

There are 3 types of square:

 (a) squares in the middle part that have 4 squares with common 
     borders

 (b) corner squares (of which there are 4) that have 2 squares with a 
     common border

 (c) edge squares, which have 3 squares with a common border

There are  36 type (a) squares.
            4 type (b) squares.
           24 type (c) squares.  

Altogether there are 64 squares on the board.

The probability that two squares have a common border is then:

  Prob(type a) x 4/63 + Prob(type b) x 2/63 + Prob(type c) x 3/63

 =  36/64 x 4/63 + 4/64 x 2/63 + 24/64 x 3/63

     36 x 4 + 4 x 2 + 24 x 3        224        1
 =  -------------------------  =  ------  =  ----
            64 x 63                4032       18

- Doctor Anthony, The Math Forum
  http://mathforum.org/dr.math/   
    
Associated Topics:
High School Probability

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