Khan Academy / Stay away.
They don't provide lessons they require you to learn. So if you have to learn something serious for exams or that kind of stuff, you better choose another website or hire a tutor.
I'm 35 and it's hard to say I'm a student, but I need some knowledge for my job, so that's why I needed those lessons to learn Electrical engineering. And once I missed several lessons because of my job.I contacted the Khan tutor and was advised to find needed lessons to run my course down. I tried to find them, but I didn't. Nothing has helped me to find them.