Thanks for your feedback. I will test it today on a classroom. I will see if the arragement of the area is ok.
That will be the best idea, maybe it's only me.
But for the challanging I'm agree with you. That's why I wrote that's a beta version. It's just 2 hours of work.
For two hours work it's really good.
If it should be more like a game I would do levels. Maybe in the first levels only whole sums and a level consist of only a few runs after another. Higher levels all kind of sums more runs. Maybe a time limit in higher levels? Bonus for the lowest numbers of notes and coins as change? Some kind of achievements like stars or medals? A High-score list could be something very motivating.