Relative pronoun whose

Whose is a relative possessive word. It is a determiner used before nouns and replaces the words his, her, its or their.

Whose can refer back to people or things. In a relative clause, whose +noun can be the subject or object of the verb. It can also be the object of a preposition.

I know a man whose son studies at Oxford. (Here whose + noun is the subject of the relative clause ‘whose son studies at Oxford’.)
It was a decision whose significance I did not understand. (Here whose + noun is the object of the clause ‘whose significance I did not understand’.)
I went to see my friend Susan, whose children I used to look after when they were small. (Here whose + noun is the object of the preposition after.)

Of which, that…of

Instead of whose, we can use of which or that…of to refer to things.

It was a meeting whose purpose I did not understand.
OR It was a meeting the purpose of which I did not understand.

Sentences with whose are rather formal. In an informal style, other structures are often preferred.

You know that girl whose brother owns a tea estate? (Formal)
You know that girl with a brother who owns a tea estate? (Informal)