Reference

Sudoku (soo-doh-koo) is a 9x9 logic puzzle to superheat the very core of your brain and to challenge your numeracy, spacial awareness and problem-solving abilities. Warning: the five levels of "Penguin Sudoku Challenge: Volume 2" may leave you smugly grinning, scratching your head, sighing deeply, snapping your pencil - and you may even throw your book at the wall. But you also have permission to frame your most difficult completed 9x9s. Good luck!