Here are the sentences converted to active voice:

1. Someone did these exercises well.
2. You must wash this dress in cold water.
3. He has made no mistakes in his composition.
4. The New Arts gallery will exhibit his painting for the first time.
5. Workers built this house 100 years ago.
6. They make these artificial flowers of silk.
7. The students are writing the lessons now.
8. Where did pirates use to hide the treasure in old times?
9. They feed these animals at the zoo twice a day.
10. French doctors are going to perform the operation.
11. Someone has made all the arrangements.
12. When did they manufacture this radio?
13. They couldn't find them anywhere.
14. Do they teach mathematics in this school?
15. Fortunately, the accident didn't damage the machinery.
16. Please wait here while they examine your luggage.
17. How do they make candles?
18. For a long time, people believed the earth to be flat.
19. Did your teacher teach you how to apply this theory?
20. People say apples are good for our health.
21. Don't let the children tease the dog.
22. People believe that red is the symbol of luck.
23. Someone must have stolen the jewelry while she was out.

If you need further assistance, feel free to ask!