How does Riot chooses a League for a summoner?

***I was just wondering how does Riot chooses a League for a player? ***

1) Does assists counts? Like when you are a support, you will mostly not have kills but with the help of your teammates you get assists.

2) How much wins does someone need to be put in on a good League ( Silver, Gold, Platinum(Maybe?) ) ?

Most of the time in promotions, i get to play with unskilled players. Bad junglers, bad supports.. and so on. And when you play with unskilled players in a promotion well that's for sure you are not going to be placed in a good League but Bronze. Is there a way to reduce the amount of bad players in a 10 games promotion? Well there is always a way other than waiting days, weeks, even months to play with good summoners in the 10 games promotion. My suggestion is that, why not let people that played in the last season play with the same Leagues they had before and up a League? ( Silver = Silver/Gold, Gold = Gold/Platinum, Platinum = Platinum/Diamond, Diamond = Diamond/Challeng ).

What do you guys think? Answer your opinions below.

• Maybe some people out there have more questions to ask.