Arrange 20 counters in the form of a PLUS(+), as you can see in the image.
Now, how many different ways are there in which four counters will form a perfect square if considered alone? Thus the four counters composing each arm of the PLUS(+), and also there is four in the center of it, form squares.
Squares are also formed by the four counters marked A, the four marked B, and so on.
How will you remove 6 counters from it so that not a single square can be so indicated from those that are still there?