Originally posted by aginis
you are blindfolded ad then 100 coins are placed on a table in front of you. 7 are showing heads and 93 show tails.
You may flip coins or sort coins, how do you create two (not necessarily equal) piles such that each pile has the same number of heads showing. (no feeling the coin to see which side its on; this is a strictly mathamatical puzzle)
Choose any 7 of the coins and slide them carefully to make a second pile (the other 93 constitute the first pile). Turn the 7 coins over. Ta-da!
[If the 7 coins were all tails then there are 7 heads in pile 1 and you have made 7 heads in pile 2 when you turned them over. If there were 2 heads in the 7 you chose then when you turn them over you get 5 heads ... the same as there are in pile 1. Etc]
I like this puzzle because there is no computer program needed (too much hassle to bother with!) and at first glance it appears that there is not enough information to solve it.
[Also - anyone know why when I type an apostrophe in this box (browser = Firefox) it opens a "find" pane?]