Calcudoku

Calcudoku puzzle (also known as Mathdoku, Newdoku, or KenKen) is similar to Sudoku, but requires basic math skills to solve.

•  The goal of each puzzle is to fill a grid with digits; 1 through 4 for a 4x4 grid, 1 through 5 for a 5x5, and so on.
•  Each digit can appear only once in each row and in each column.
•  Additionally, the grid is divided into heavily outlined groups of cells called "cages".
•  Each cage has an assigned mathematical operation and a target number, that appear in the upper left corner of the cage.
•  The numbers in the cells of each cage must produce the specified target number when combined using the specified mathematical operation for that cage.

A very addictive game that helps you practice and improve your math skills.