My grandmother, whose 75, still goes swimming every day. (original)
My grandmother, who is 75, still goes swimming every day. (correction)
We have a dog which name is Groucho. (original)
We have a dog, whose name is Groucho. (correction)
Yes, both of the corrections are correct.
We use whose like this:
My grandmother, whose dog is named "Groucho", still goes swimming every day.
and we use who is like this:
Groucho, who is my grandmother's dog, goes swimming every day.
In sentences like,
We have a dog which/whose name is Groucho
Your teacher or book wants to teach that we should use whose to stand for people, animals, and things. If we want to use a pronoun to stand for a person, however, we should not use which.
We have a horse whose name is "Star."
My brother build a barn, which is Star's home.
Note that who's is the contraction for who is, so we could also use Groucho, who's my grandmother's dog ....