Multiplying my age by 6 then subtracting 6 produces the same result as subtracting 7 from my age then multiplying by 7. How old am I? On my broken calculator with keys +- x =, the only functional number is 7. How can I get 34 to appear in the readout? A country mints four denominations of coins, in whole numbers of cents. It takes four of these coins to make 21, or 24, or 25, or 26. What are the denominations of the coins? These andalmost 300 other mathematical puzzles appear in this original collection, devised by world-renowned mathematicians, puzzle creators, and devoted puzzle lovers. A unique puzzle project, it unites the efforts of a dozen authors, including software engineer Andrea Gilbert and Bram Cohen, author of the P2P BitTorrent protocol. Seventeen different types of challenges include 3-D puzzles, chess puzzles, connections, dissections, foldings, geometrical puzzles, logic problems, matchstick puzzles, mazes, moving pieces, number puzzles, put-togethers, strimko, sudoku, visual puzzles, weightings, and word puzzles. The difficulty level of each puzzle is marked by stars, ranging from2 to 5. Average difficulty level is about3 stars, promising puzzle enthusiasts many entrancing hours of solving and enjoyment.