Ответ:

10. If Robert calls his friends, they will ask him to their party.

11. If Robert goes to the party, he will drink too much beer.

12. If Robert drinks too much beer, he will need a ride home.

13. If Robert needs a ride home, he will have to stay until the end.

14. If Robert stays until the end of the party, he will get home very late.

15. If Robert gets home late, he will go to bed late.