A third common way of showing possession in English
is through using of. One way of forming possessives
of this type is with a noun phrase, one, or a number or
quantity followed by of and then a possessive pronoun:

Bob is a friend of his.
Mrs. Smith is a neighbor of ours.
Angela and Julie are friends of theirs.

Is this painting one of yours?
If you don't have a pencil, use one of mine.
Amy has three dogs. Is that dog one of hers?

I borrowed two of Bob's books and several of yours.
The boss like several of their ideas and all of ours.
Jim ate his lunch and also half of mine.

Special Note:

Notice these differences in meaning:

Bob is Jim's friend. / Bob is a friend of Jim's.

In the first sentence, we know only that Jim has one
friend--Bob. In the second sentence, we know that
Jim has several friends, and Bob is one of them.

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Mrs. Smith is our neighbor. / Mrs. Smith is a neighbor
of ours.

The first sentence states only that we have one neighbor--
Mrs. Smith. The second sentence states that we have
several neighbors, and Mrs. Smith is one of them.

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Angela and Julie are their friends. / Angela and Julie
are friends of theirs.

In the first sentence, we know only that they have two
friends-- Angela and Julie. In the second sentence, we
know that they have several friends, and Angela and Julie
are two of them.