Addition is one of the simplest and earliest mathematical tricks taught. Everyone ends up adding two numbers every day for some reason or the other. There couldn't possibly be anything more basic than this.
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Griddition Alternatives
Griddition is described as 'Math puzzle game in which you must test your addition skills across over 190 levels of increasing difficulty and enjoyment, earning trophies and bragging rights along the way. Think of Griddition as Sudoku for the 21st century' and is a Puzzle Game in the games category. There are more than 10 games similar to Griddition for a variety of platforms, including Android, iPhone, Android Tablet, iPad and Mac apps. The best Griddition alternative is Gauguin, which is both free and Open Source. Other Griddition like games are Sudoku.best, Sudoku by Pink Pointer, NumberDrop and Sisiban.