Installed it out of curiosity, filled with adds, regular banner and full screen ones. Looking at the contents briefly, not sure what value it really brings.

Edit: sorry that's not really and answer to the OP question, just wanted to warn others

I don't think there are any open source apps exactly like this yet. You could try this app in combination with a book about mental math tricks.

No idea what this program is for, but from the practical use point of view, the example from the image is horrendous: 35x11 is calculated trivially as 35x10+35. Not that craziness.