The book contains 200 Calcudoku puzzles 6x6 at the level of Hard. All riddles have only 1 solution. Each puzzle consists of a grid containing blocks surrounded by bold lines. The object is to fill all empty squares so that the numbers 1 to N (where N is the number of rows or columns in the grid) appear exactly once in each row and column and the numbers in each block produce the result shown in the top-left corner of the block according to the math operation appearing on the top of the grid. In Calcudoku a number may be used more than once in the same block.