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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?

enter image description here

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I think there are

21 squares

Counting

9 of this variety (2 on each leg plus the central one)
enter image description here
4 of this variety (can move this shape right, down or right+down). enter image description here
These 4
enter image description here
And these four
enter image description here

I would remove these counters

enter image description here

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  • $\begingroup$ Checked you might ignored the square of length $2\sqrt{2}$ :D $\endgroup$ – Conifers Sep 17 at 11:04
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    $\begingroup$ @Conifers Yes, you are right, thanks. $\endgroup$ – hexomino Sep 17 at 11:09

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