Mémentis - educational game

Mémentis is a free online game of the category educational games.

Mementis is an excellent game of mental calculation developed by two French, ideal for improving its capacity in calculation and exercising your brain in a way both useful and entertaining.
The goal is to find missing numbers in white boxes by doing mental arithmetic on the four operations. There is no notion of priority in the division or multiplication. Six levels of difficulty are available and beginners course will start by level 1, which is missing one digit per line.
Notes: At times, you'll have to calculate backwards and it is from the figure that as the result of the operation and from right left by calculating each operation without forgetting to reverse the sign of operation. A "+" becomes a "-" a ":" becomes an "x".

This game is not interactive yet, paper and pencil are needed. It is preferable not use the calculator, so Mementis will help you improve your skills in mental arithmetic.
Mementis website offers other grids.