https://www.mathacademy.com/ is probably the most popular HN response.

It's expensive at $50 per month, but it seems highly thought of if you look at Algolia (https://hn.algolia.com/?q=math+academy).

I haven't used it myself, but I am evaluating it for my kids to use.

It's too early to give feedback on that front, though.

This is a great resource for self-learning mathematics: https://www.susanrigetti.com/math

Start here: No BS guide to Linear Algebra https://a.co/d/6em7tXE . There’s also a No BS guide to Math/Physics which goes through calculus and geometry. This is freshman level content. I read this on the couch next to my wife act age 37 for about 20-30 minutes a night, a few times per week and got through it in six months. You will need to work some of the exercises, i used a lap desk and clipboard which I store next to my spot on the couch.

Through most of my life, I believed that I was utterly incapable of doing math and mostly just kind got by in high school, didn't really think I would need it that much. Then I took a reality check the first time I attempted the equivalent of my country's SAT test. At the time, I got in my head that I wanted to be a computer engineer and that required at least above average grades in math and physics so that I could have a chance. There really isn't much I can offer in advice, besides: diligent study. That was what got me by. Sitting my ass at the desk everyday for hours on end, starting from first principles (and I mean first principles, like basic arithmetic first principles) and doing as many practice problems as I could. I probably did too many of them, but getting them right was the only thing that brought me any confidence that I was learning. I did that for about a year and, while it didn't turn me from math illiterate to math genius, it gave me enough of a foundation to get into college and have a easier time than my peers on subjects like calculus. In your case, you can take a lighter approach, since it isn't urgent by any means, and you probably won't have as much time as I did. I recommend setting a time whenever you can to study, ideally it would be around two hours, but I find it more important be consistent about it. If you are anything like I was, you may have some knowledge on math, but its akin to Swiss cheese: full of holes. That being the case, start from the earliest point it makes sense to you. I was very paranoid at the time, so I started from basic arithmetic and, although that may not have been necessary, starting from scratch gave me the confidence that I wasn't missing anything going forward. Mathematics is a cohesive structure, where the higher elements are build upon the lower foundations, you cannot get very far without having a good grasp on the basics. As to learning resources, there are many online for free (Khan Academy[1], Paul's Math Notes[2], many introductory books at the Open Textbook Library[3])

Lastly, try to have fun with it. Back when I started the journey, I was to focused on what I could get from learning math (that is, getting into college and then, hopefully, a job), that took out of the experience a bit and the only thing that kept me going, though I didn't recognize at the time, was the joy of learning something that I believed my whole life to be unapproachable, at least by me. So be patient, if you bang your head enough times against this wall, I can attest that it eventually starts to show cracks.

Good luck on your learning journey.

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[1] https://www.khanacademy.org/ [2] https://tutorial.math.lamar.edu/ [3] https://open.umn.edu/opentextbooks/subjects