The question as it appears in the 02/06 edition of Specs & Techs from GlobalSpec:
You have an incomplete deck of cards and are asked to randomly select five percent of the set. You follow a pattern for the selection  select the first card, miss one, select the next, miss two, select the next, miss three, etc. Following this method, the last card you selected is also the last card in the deck. And the number of selected cards is exactly five percent of the total cards in the deck. How many cards were selected?
Update (02/13/07 8:52 AM): And the Answer is....
Let n be the number of cards picked. Because n is five percent of the total, then the number of cards in the deck is 20n. The cards were picked following the following pattern:
So the number of the final card picked is , but this is precisely the last card in the deck. Therefore,
The nonzero solution to this equation is:
Therefore, you picked 39 cards that make up five percent of the deck.

Re: Deck of Cards: Newsletter Challenge (02/06/07)