Since making the post below, I have found an even better way to learn math starting with simple algebra through precalculus--the precalculus class sponsored by Arizona State University on edx.org at https://courses.edx.org/courses/course-v1:ASUx+MAT170x+2T2017/course/ featuring the Aleks tutoring software. Aleks, like schoolyourself.org, tests your skills and keeps track of those you have learned, but it is has more material and is more comprehensive. The course is free unless you want a certificate.
If you really want to start with a lot of elementary stuff, like addition and subtraction, I suggest you look at
which I started using yesterday.
I wanted to review math from the ground up myself. I completed in-person classes through Algebra II and Trigonometry many years ago but have forgotten a lot and have been having difficulty getting a good start on calculus as a result.
The site at schoolyourself.org is interactive and adaptive. I was able to zip through most of the early units on basic arithmetic in several hours. The site lets you turn the sound up to double speed. If it detects that you know the material there is very little repetition, but it offers help and gives you additional explanations, practice, and review where it detects you are having difficulty. The topics listed go all the way through calculus and statistics, though I have not gotten that far yet.
Even in the early material I found things to ponder, like the significance of the fact that division by zero is undefined and of the apparent asymmetry between the way positive and negative numbers work, and why computational tricks I learned years ago, like cross-multiplication to compare fractions with different denominators, actually work.
The site was apparently bought out a couple of years ago, and does not seem to have developed since then, but what I have seen there so far is the best of its kind I have been able to find.