One of my uncles showed me that long ago and solved it exactly like it your first post.
So I showed it to my math teacher ( Phd in math... he's a fucking genious i don't know why the hech he doesn't teach in a uni...). And he exmplained. When take 10a and remove a from it, there's a spot in infinity that actually stays. Basically a=0.99~, so 10a=9,9~, 10a's infinity is delayed
ohaz's demonstration quite nice too didn't expect it