I'm most of the way through my semester of mathematics class for my Bachelor's degree, and I found myself "comically unprepared" for Calculus (seriously, that's my Khan Academy biography at the time of writing). I've been using Khan Academy to great effect to catch up on things like Algebra, Trig, and so on and it's been a huge help to me. When they had their fund raising campaign just before Christmas I slipped them a few bucks because they've seriously been helping me out a lot.
Yesterday or maybe the day before, I noticed that I'm almost complete with the entire mission for "Algebra Basics", and I wondered what would happen if I 100%ed it. So I went and "practiced" all the things I hadn't touched, and began banging out mastery challenge after mastery challenge. I reached the final four skills at level two, and had five tasks... This is it!
I smashed out all four skills, but the second last question was something I'd already mastered - calculating the slope of a line from two points.
I had two sets of points, something like (-20, 75) and (130, 25) and so I wrote the problem down and set about solving it (no biggie, I've already mastered this skill!). This is more or less exactly what I wrote on my paper (y values might be wrong, I know the x values aren't as they're seared into my brain):
Now, if you're paying attention (like I wasn't), you'll notice that this isn't correct for the values given. So because of this, a skill I'd already mastered got returned to "not-mastered" status, and I'm at 99%. It also won't give me any mastery challenges at the moment, so I'm not sure how to get it to 100% and it's really bothering me.
God damn it.