2015 год. Start off easy, with medium-level Sudoku, move on to the harder grids, and finally try your luck with the tear-your-hair-out very, very HARD conundrums. Even though you're filling in the boxes with numbers, there's no math skill required; just pure logic. The introduction provides basic strategies, explaining how to exclude possibilities and leading you through a sample puzzle in great detail. By the time you've made it through this collection, you'll have sharpened that gray matter and built those mental muscles.