Mental Maths Practise Game
It’s not the most beautiful script, but it works! It has definitely made my mental maths much faster. Note it doesn’t catch every exception, only the obvious ones, so you can easily make it crash if you actively try to - but why would you?
Features addition, subtraction, multiplication, division, random (a combination of the four), and a dividend test. Each can take custom numbers, tolerances, and you can vary the number of rounds. A timer and score is built in.
The embed doesn’t seem to work, so you can visit Repl.it directly and play the game there: