Thank you for playing! I would have liked to work on the UI more, but I was in a crunch since I only had about 24 hours to work on the game due to unavoidable commitments. When I update after the jam is over, I'll be sure to add a tutorial or similar :)
The division by zero... Internally, it causes the question variable to become NaN (Not a Number), and so the NaN checking for whether the user has selected a number or not takes over. I would have loved to add a little Easter egg, like making the number equal to infinity, but again, didn't have time.
I never thought about the problem of the player not knowing to translate tally marks... Where i live, tally marks are taught at school and I never realised it might not be the same elsewhere.
Once more, thank you for your valuable feedback, and I hope you have more fun with the game when it's in a more polished state in the future!