6. Both of my parents are from London. - My grammar book says it's possible to omitt 'of' in this kind of sentence. But what about №3?
Yes, it's ok, also in 3.
7. I tried twice to phone George but both times he was out. - I am not sure whether I'm correct or not.
This is starting to look like homework. We don't do that.
8. It was a boring film. It was very long too. --> The film was both boring and long. - Not sure about it. Sounds OK.
9. Is that man's name Richard? Or is it Robert? It's one of the two. --> That man's name is either Richard or Robert. - I might be mistaken. Correct me please if I'm not right.
10. You can phone me at any time during the evening. I'm always at home. I've chosen a word 'any'. Is it a proper one? Yes.
Thanks in advance!
We don't correct or do homework, but we do answer questions that learners are not sure about, and are interested in. A list of 10 questions that require answers after a class on that subject looks very much like homework.
In the case of homework, you should do your best, let your teacher correct it, and then, if you're still not sure, post that question. Otherwise, we would be overwhelmed doing other people's work.