A friend came up to me today with a deck of cards, and put these 6 into a group:

2 3 5 7 J K

The other 7 values went into another group:

A 4 6 8 9 10 Q

He then challenged me to find the reason for the groupings.

"That's easy," I said. "The first group is primes!"

"Well, yes, but not what I had in mind," he replied, with a twinkle in his eye. "This property isn't mathematical!"

I thought for a while, then exclaimed, "O!"

What was the property?

  • 1
    $\begingroup$ I can only count 7 values in the second group. Typo, perhaps? $\endgroup$ – ZanyG Dec 26 '19 at 4:19
  • $\begingroup$ Is "exclaimed O!" an intentional hint? $\endgroup$ – Scratch---Cat Jan 11 '20 at 11:49
  • $\begingroup$ @Scratch---Cat indeed it was. $\endgroup$ – Quintec Jan 12 '20 at 1:52

In the first group the numbers and letters have their strokes ending with loose ends, not forming circles or closed loops, while the second group has those properties (circles in 6, 8, 9, Q and also 0 in 10, and both 4 and A have triangles)

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    $\begingroup$ Could you use a spoiler tag (type >! at the start of the line) to hide your answer? Also, please explain your reasoning a little more so that it is easier to understand. $\endgroup$ – ZanyG Dec 26 '19 at 4:15
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    $\begingroup$ @CG. In symbols of the second group, you can use the MS Paint fill tool to fill only a small part in the inside of the letter/number. $\endgroup$ – JiK Dec 26 '19 at 13:18
  • $\begingroup$ Forgive me if I'm mistaken but wouldn't the K be in the second group? $\endgroup$ – im_so_meta_even_this_acronym Dec 27 '19 at 3:35
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    $\begingroup$ @im_so_meta_even_this_acronym No, All edges in K are open, there's no closed shape such as in A,B,D,O,P,Q,R. $\endgroup$ – sudhackar Dec 27 '19 at 6:43
  • $\begingroup$ Does "exclaimed O" make sense? $\endgroup$ – Scratch---Cat Jan 11 '20 at 11:48

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