Khan Academy is not bad if you like their style of presentation, but as another comment noted, the number of exercises is too few. Unfortunately, I have yet to find a good recourse with focus on exercises, so I tend to create them myself for courses I teach. If you (or any other for that matter) are interested, I can upload a work sheet somewhere.

There properly are tons of sites providing work sheets, but the focus then tends to be on K-12 education and they are often to simple IMO.

For university the importing thing is often manipulating say fractions rather then "calculating" with fractions.

Tangentially, would there be any interest in a service providing this? Pay say 2$ or something and get 50 exercises in the basics with an eye towards university with sample solution? That space is properly swamped but maybe there is a niche? I often think that there is room for textbooks of this short but then again, that market is also swamped with big publishers.