7
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I took a standard 52-card deck and split it into two piles.

Pile 1:
Clubs: A 3 5 6 7 8 9
Diamonds: 7
Hearts: A 3 5 6 7 8 9
Spades: A 3 5 6 7 8 9

Pile 2:
Clubs: 2 4 T J Q K
Diamonds: A 2 3 4 5 6 8 9 T J Q K
Hearts: 2 4 T J Q K
Spades: 2 4 T J Q K

What was my criterion?

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10
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The criterion was:

Separate the deck into two piles: one pile contains cards with a front face that is symmetric, the other pile contains those which are not symmetric. (All cards are symmetric on the back)

Note:

Rotational symmetry. (you can turn the card 180 degrees and it will look the exact same)

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  • $\begingroup$ Isn't that only literally true with the diamond suit ? Also 7 isn't symmetrical $\endgroup$ – moonbutt74 Sep 1 '15 at 16:28
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    $\begingroup$ For the diamonds, the only one that's not rotationally symmetric is the 7 because the diamond itself is symmetrical. The other suits end up with a non-symmetric symbol in the center of a lot of the cards. $\endgroup$ – Kingrames Sep 1 '15 at 16:30
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    $\begingroup$ That was exceedingly fast... I'll wait a bit before accepting, so as not to discourage other readers... :) $\endgroup$ – João Mendes Sep 1 '15 at 16:30
  • $\begingroup$ I played a lot of friendly card games as a kid. There's a lot of fun trivia in that card deck. $\endgroup$ – Kingrames Sep 1 '15 at 16:31

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