Some Japanese guys state that the best way to train our brain is to perform very simple calculations. However, it's not enough just to remember what is 2 times 3 from time to time - you gotta do it hard.
Below you will find 100 very simple exercises which hardly challenge even your arithmetic skills.
These are trivial, but the large number of them makes it not that easy to avoid occasional mistakes and finish all of them without feeling tired. Maybe that indicates some positive activity going on in our brains.) Well, at least it makes me sum and multiply faster.
Try to do all of them as quick as you can.