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Squares in a cross (Posted on 2006-05-10) Difficulty: 3 of 5
In the diagram formed of twenty Os below, in how many ways may four be selected such that they form a square? What is the least number of Os that may be removed such that none of these squares remains intact?
O O
O O
O O O O O O
O O O O O O
O O
O O
Note: the horizontal and vertical spacing should be equal.

See The Solution Submitted by Jer    
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Solution Solution | Comment 1 of 5

I've found 19 squares:

oo
oo     = 9
 o
oxo    = 4
 o
  o
  x
oxxxo  = 4
  x
  o

  o
oxxx
xxxo   = 2
 o


You can get rid of 5 o's (replaced with X) to eliminate all of these possibilities like so:

  oo
  Xo
oXooXo
oooXoo
  Xo
  oo

  Posted by tomarken on 2006-05-10 08:55:33
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