Keen is a free puzzle game for Android that challenges the player to fill a grid without overlapping numbers, while also satisfying the math operations in each section. This game is similar to Sudoku and is sometimes known as KenKen, Calcudoku, or Mathdoku. The app offers seven different grid sizes and five different difficulties, including a multiplication-only option.

One of the best things about Keen is that it provides an unlimited number of levels, so you can play as much as you want without getting bored. Additionally, the app has an undo function and allows you to save and continue your game at a later time. It is also worth noting that Keen is free, contains no advertisements, and is open source.

However, the app does not include a tutorial, so if you are not familiar with this puzzle type, you will have to learn via experimenting or Google. Also, high difficulty multiplication-only levels can take some time to generate.

Overall, Keen is a great puzzle game for those who enjoy Sudoku or similar games. With its unlimited levels, different grid sizes and difficulty levels, and a multiplication-only option, this app is sure to keep you entertained for hours.