If you are talking about coolmath.com then the hardest game is called the world's hardest game and it is about trying to take a red ball across to the other side without getting hit by the blue balls and collecting the yellow ones!
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Maybe any app that is a math game is cool, since practicing math is great. Super spiro, Rocket Math, Snail Bob, My Math and Math Bubble Popper are all cool apps which test simple math and even some geometry.
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its really easy! but if it doesen't work, click this link:Cool Math Games - Free Online Math Games, Cool Puzzles, Mazes ...I hope this helps:)
Cool Math Games is an online web portal that serves as a platform for Flash and HTML web browser games. Cool Math Games first went online in 1997. The site hosts only games that they believe are non-violent and educational.
There is plenty of websites that offer to play cool math games online. The best games are at Big Fish. One can find anything he likes at Big Fish. One can also buy a game there.