I let you what I know, sorry if it's not a complete answer:

In 5v5, everyone's elo is important, because, Riot will match you with teams according to your team elo. And now, it doesn't matter if you win the 5 games, what matters is if you win against to high elo teams.
According to this, if you win 5 games against 5 Bronze III teams you should appear in Bronze III-II.
But if you win only 3 games against Gold 3 and you lose 2 against platinum or Gold I ,per example, you should appear in Gold for sure.

No matter the quantity, but quality.

Sorry for my bad english, trying to do my best.

Here's what I know, I'm sorry if it's not a complete answer.

In 5v5, everyone's elo is important, because Riot will match you against a team with similar elo. This is important because it's not just winning that matters, but who you win against.

For instance, if you win 5 games against 5 Bronze III teams you'll be placed into Bronze III-II. But if you win 3 games against Gold 3 and lose twice against Platinum or Gold I, you'll likely be placed into gold.

In short, it's not just the quantity, but the quality of wins that matters.