Sure! Here are the transformed sentences:

16. That was a silly thing to say!
→ What a silly thing to say!

17. We’ve run out of petrol.
→ There is no petrol left.

18. It takes six hours to drive from here to London.
→ It is a six-hour drive from here to London.

19. He was sorry he hadn’t said goodbye to her at the airport.
→ He regretted not saying goodbye to her at the airport.

20. They failed to find out a solution to the problem.
→ They didn’t succeed in finding a solution to the problem.

21. Most stores will accept a credit card instead of cash. (ALTERNATIVE)
→ Most stores will accept a credit card as an alternative to cash.

22. You can avoid tooth decay by brushing your teeth regularly. (PREVENT)
→ Regular brushing of your teeth can prevent tooth decay.

23. Some of Peter’s expressions make me think of my brother. (REMIND)
→ Some of Peter’s expressions remind me of my brother.

24. When do you think this bridge was built? (OLD)
→ How old do you think this bridge is?

25. That old typewriter is not worth repairing. (POINTLESS)
→ It is pointless to repair that old typewriter.

26. They share a lot of hobbies and interests. (COMMON)
→ They have a lot of hobbies and interests in common.

27. This train should have left 30 minutes ago. (MEANT)
→ This train was meant to leave 30 minutes ago.

28. The president arranged for me to use his chauffeur-driven car whenever I liked. (DISPOSAL)
→ The president put his chauffeur-driven car at my disposal whenever I liked.

29. The painting is worth $25,000. (VALUED)
→ The painting is valued at $25,000.

30. This passport can be used in most countries.
→ This passport is valid in most countries.