There are several books on a bookshelf. If one book is the 4th from the left and 6th from the right, how many books are on the shelf?

Three wise men are told to stand in a straight line, one in front of the other. A hat is put on each of their heads. They are told that each of these hats was selected from a group of five hats: three black hats and two white hats. The first man, let’s call him A, at the rear, can see both other men and their hats. The second man, B, in the middle, can see only the last man and his hat. The last man, C, standing at the front of the line, canâ€™t see either of the men behind him or their hats.

None of the men can see the hat on his own head.

When A is asked if he knows the color of the hat he is wearing, he says no. When B is asked if he knows the color of the hat he is wearing he says no. When C is asked if he knows the color of the hat he is wearing he says, “Yes, my hat is black.”

He is correct. How did he come to this conclusion?

You have two hourglasses, a 7 minute one and an 11 minute one. Using just these two hourglasses, how can you accurately time 15 minutes?

A book costs \$1 plus half its price. How much does it cost?