Alexander Bogomolny

Cut the Knot is a collection of probability riddles that have been carefully crafted to test the intellect and develop mathematics and logical reasoning abilities. These problems and their answers, which were first published on cut-the-knot.org, are the result of the combined efforts of brilliant minds from all around the world. 

Alexander Bogomolny presents these selected puzzles in a topical progression.  Before reading his suggestions, give them a shot on your own. Just like it was in the case of Alexander the Great, the non-trivial, unexpected solution may turn out to be exactly what you require.