Sure, here are the sentences changed into passive voice:

1. He will be sent abroad to study.
2. This room was used by her.
3. My bike was stolen yesterday.
4. A letter was received by him from his parents.
5. A new house will be bought by her.
6. My house isn't being built.
7. The math problem could be solved easily by him.
8. The floor is cleaned by me every day.
9. This dress was bought by my mother this morning.
10. This exercise was done by her two days ago.
11. These flowers are watered by my mother every morning.
12. This house is going to be bought by them.
13. His parents will be visited by him next month.
14. The secretary wasn't phoned by the director this morning.
15. The window was broken by Nam yesterday morning.
16. The test was being done by them at 3 p.m. yesterday.
17. Hoa was invited to Nam's birthday party last night.
18. The dinner is being prepared by her mother in the kitchen.
19. Our teeth should be cleaned twice a day.
20. The problem has been explained carefully by our teacher.