Pronombres posesivos reemplazando «adjetivos posesivos + nombre»

1. This is our room > This (room) is ours. This is their car > This car is theirs too. You’ve got my pen. You’re using mine. Where’s yours?
2. A friend of mine: «one of my friends.» A sister of hers: «one of her sisters

[La expresión «of mine» significa «one of my,» etc.]