The AK47 problem

Cyberspace Rendezvous pointed me to a Wikipedia list of cognitive biases.
This puzzle challenges these biases.

You see four cards: 'A', 'K', '4' and '7'.
All cards have a number on one side and a letter on the other.
Check if the following proposition holds for each card, turning as few cards as possible:
"If there's a vowel on one side, then there's an even number on the other side."
How many cards to you turn, which ones, and why?

The answer, as always, is in rot-13.

Rirelbar (pbeerpgyl) purpxf 'N'.
Gur arkg pneq vf uneqre.
Fbzr crbcyr pubbfr 'X'. Jebat: gur cebcbfvgvba vf abg nobhg pneqf jvgu pbafbanagf.
Fbzr crbcyr pubbfr '4', zvfgnxrayl nffhzvat gung gur erirefr bs gur cebcbfvgvba fubhyq ubyq.
Gur pneq lbh arrq vf '7': vs vg unf n ibjry, gung snyfvsvrf gur cebcbfvgvba.
N terng rknzcyr bs pbasvezngvba ovnf.

Posted by cronopio at 03:06 PM, March 30, 2006