First of all, do not spend two days on problems. It's sometimes very tempting to be "disciplined" and never give up on a problem. I used to put monumental energy into problems out of my reach, and I learnt very little, even though I did in fact solve them. If you're stuck for 30 minutes with absolutely no clue — check out the editorial. Use the hints. Check out the solution. Code the thing yourself until you get AC. And move on to another problem. Discipline is about doing the right thing. It's not about looking/proving you're tough, even to yourself.

Second of all, if we're talking CP math, just solve problems. div3 D might be too difficult for someone rated 1000, and that's okay. Be humble and solve something simpler. You'll gain confidence and skill there, get some pattern recognition going, learn to think in terms of problem solving. If you suck at something, the only way to get better is by doing it.

I'd just say solve more problems, I'd argue this problem isn't extremely mathy (only algebra is required, and there's also a solution with very minimal math), and you could probably gain the intuition to solve this without directly focusing on math problems.

Also agreeing with the other commenter, div3 D might be a bit difficult for a 1000 rated user and that you shouldn't spend 2 days on a problem (though I've seen conflicting advice on whether to read the editorial or just come back to it later)

I would reccomed doing counting and probability on Alcamus on AOPS website, it’s free