You have a stack of 23 cards. Each card has an image of a cross on one side 

& an image of a circle on the other side. You know that 14 cards in the stack are 

lying with the cross up, and therefore 9 cards are with the circle up. However,

you do not know in which order the cards are. Moreover, you are in completely dark 

room,so you cannot see anything. 

The Question: How can you, without seeing anything, divide the stack of 

cards in two smaller stacks, in such a way that both stacks have the same 

number of cards with a cross up?

A Hint : You can turn cards round

The Answer: 

Take 14 cards from the stack. Of these 14 cards, k cards are with the cross up 

and therefore 14-k cards are with the circle up. In the remaining stack of 9 

cards are 14-k cards with a cross up.

                                      Then turn the stack of 14 cards round. 

Now there are 14-k cards with a cross up in both stacks.Note that we do not 

know exactly how many cards are lying with a cross up in both stacks (or how

many cards are lying with a circle up). We only know that both stacks have 

the same number of cards with a cross up.