In the diagram formed of twenty O’s below, in how many ways may four be selected such that they form a square? What is the least number of O’s that may be removed such that none of these squares remains intact? Note: the horizontal and vertical spacing should be equal.

(In reply to Solution by tomarken)

You are missing a couple of squares:

  ox
  xx
xxxxxo
oxxxxx
  xx
  xo

so it seems you´d have to get rid of an extra "o".

Posted by Federico Kereki on 2006-05-10 09:08:52