Here are the suggested answers for the sentences:

11. **Is Sarah working** this week? - No, she is on vacation.
12. She **has been learning** English for three years.
13. I’m hungry. I **haven't eaten** anything since breakfast.
14. We **have known** each other since we were at high school.
15. I **have forgotten** my key. I can’t unlock the door now.
16. Eric is calling his girlfriend again. That is the third time he **has called** her this evening.
17. The police **have arrested** two men in connection with the robbery.
18. You **received** any letters from your parents yet?
19. My father **has not smoked** for 5 years.
20. John **has washed** his hands now. He just **repaired** the bike.