20. She often does the housework after doing exercises.

=> When does she often do her homework ?

21. They are planting some roses in the garden.

=> How many roses are they planting in the garden ?

23. My favorite subject is Math.

=> What is your favorite subject ?

25. I often listen to music in my free time.

=> What do you often do in your free time ?
 

27. She learns about computers in computer science class.

=> What does she learn about in computer science class ?