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?
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.
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 20060510 08:55:33 