I present to you a deck of 4 cards. Each card has on one side a letter of the alphabet, and on the other side a single digit from 0-9.

I propose a hypothesis that may apply to this deck:
If the letter is 'E', then the number on the other side is '4'.

I then drop the 4 cards on the table, and you see: 'B', '7', 'E', '4' (on the respective cards).

Which of the 4 cards must you turn over to verify or disprove my hypothesis?

What card would be a counterexample? Only one with an "E" on a face, and something like a "7" on the other face -- in fact, anything but a "4".

There are two possible "E-7" cards -- the "7" and the "E", so you must turn them over.

Posted by e.g. on 2004-12-16 10:52:25