Now that you mention it, it was probably also part ADHD hyperfocus that I managed to get so high with 0 prior knowledge (no tutoring, no classwork, 0 clues) haha ;; my mistake!
But my point is just that I definitely don't see why the average person can't do decently with 20 days to go! Harsh time management and focusing on key areas is the key here (it's what I did), I'm sure they can hone themselves in if they try! 90% is definitely a bit ambitious, but it's still possible to do decently regardless (I don't imagine OP would be aiming for 90% if they're aware that they know 0 chemistry or physics, anyways)! Weekends are a free ~10 hours of studying per day (accounting for breaks and sleep time here), which should give a decent boost.
I'm no physics student, so OP is on their own in that regard, but I can definitely say chemistry isn't too bad to cram if you're efficient and actually focusing when you study
