[WarOnPrivacy]

> Wait, how can algebra not depend on arithmetic? Something simple like x+20=3x+8.

If your Q could be restated as: How can a student struggle with one while excelling at the other?

The answer is: When we learn the answer we'll be able to help more dyslexic kids than we are now.

I failed basic 3rd grade math tests for so long they finally quit testing me. Eventually, I was the only one of those kids doing algebra in the 6th grade.

My longish life is loaded with similar discontinuity-of-ability. Adult me obfuscates it so well I rarely experience other people's incredulity. Grade school me could have put that to good use.