How does your MMR work

I bought an account from someone in silver v and he had a shitty win rate, but anyhow I'm winning atleast 75% of my games but I get 5 lp per win
and when i loose i loose 20 lp WDF?

If he has a shitty win rate with alot of games in silver 5. He has most likely made his MMR bronze. Use this site to determine your MMR MMR ::

He probably kept losing too much in Silver V getting his account clamped.
Gonna be a while before you can get it back to normal gains.