1.My friend's son
2.My friend's sons
3.My friends' son
4.My friends' sons
5.Our teacher's book
6.Our teacher's books
7.Our teachers' book
8.Our teachers' books
9.My sister's friend
10.My sister's friends
11.My parents' friend
12.My parents' friends
13.The doctor's name
14.This student's dictionary
15.This student's dictionaries
16.These students' dictionary
17.These students' dictionaries
18.Petr's family