20 coins are placed as shown in the image below. Now you can
draw squares where all corners must be at the position of a
coin (two squares are already drawn). Find all squares (hint:
there are more than 20 squares).
What is the minimum number of coins one must remove, so that
it is not possible to draw any square ?
Correct answer: 6
Explanation:
There are altogether 21 squares (see images below). At least 6 coins must be removed (one example is shown at the bottom of the image below).
