I thought a student I work with would find it useful to improve their fluency at addition, subtraction and multiplication of small signed integers. I also thought that there would be many websites doing this job. But after a non-trivial (but non-exhaustive) search, I didn't find anything which did quite what I wanted. There were games which would do addition and subtraction, and games which would do multiplication, but I didn't come across one which mixed the operations. And a lot of them were dressed up in games which often looked good, but were quite heavyweight.
I also wanted to experiment with replit's AI-driven agent, so used this as a test case, describing what I wanted, and iterating via to and fro with the agent. I was reasonably impressed with what it managed, although it claimed to fix a bug when in fact it hadn't, and I ran out of free credits pretty early in the development process. So I continued working on it manually. I like the way replit gives you a Git history of the agent's work, which I've preserved in this repo as the early commits.