1.the assistant of the director. Te director's assistant
2.the nephew of Sir Charles.  Sir Charles's nephew
3.the birthday of my boy-friend. my boy-friend's birthday
4.the duties of men-servants.  men-servants' duties
5.the last letter of my mother-in-law .  my mother-in-law's last letter
6.the recent statement of President Bush. President Bush's recent statement
7.the early poems of Robert Burns .  Robert Burns's early poems
8.an interval of ten minutes .  ten minutes' interval
9.a distance of five miles.  five miles' distance
10.the crew of the ship.   the ship's crew
11.the population of England. England's  popullation
12.for the sake of economy. Economy's sake
13.the mother of Mary and John  mary and John's mother (если мать у них общая)
14.the rooms of the daughter and the son. Daughter's and son's rooms 
15.the new house of the Parkers.  Parkers' new house
16.the latest record of the champion. Champion's  latest record