Ask HN: Coderbyte for Math?

I enjoy doing Coderbyte's coding challenges (I try to do one a day).

They're (mostly) easy enough to not take too long and hard enough to enjoy the dopamine release once you solve the challenge.

I wonder if anyone here knows of a good "Coderbyte for Math challenges"?

I briefly searched for it, and the closest thing I found is this horror: https://www.mathopolis.com/questions/day.php

Also it would be great if it had an iOS app (not a necessity, but would be a nice bonus)