21. You didn't tell us the reason. We have to cut down our expenses for that reason.

=> You didn’t tell us the reason for which we have to cut down our expenses.

28. This is Mrs. Jones. Her son won the championship last year

- This is Mrs. Jones whose son won the championship last year.

29. Rod Lee has won an Oscar. I know his sister

- Rod Lee, whose sister I know, has won an Oscar.