Whose bags are they?
→ Who do these bags belong to? (Correct)
Let’s go shopping.
→ Why don't we go shopping? (Correct)
There are twenty students in his class.
→ His class has twenty students. (Correct)
This film is very interesting.
→ This is an interesting film. (Correct)
These cars are very modern.
→ They are very modern. (Correct, but lacks the reference to cars. Full form: "They are very modern cars.")
Who does this computer belong to?
→ Whose computer is this? (The suggested sentence "Whose computer does this belong to?" is incorrect, so it should be changed to this correct form.)
It takes her twenty minutes to practice English every day.
→ She spends twenty minutes practicing English every day. (Correct)
That is a very intelligent boy.
→ He is a very intelligent boy. (Your answer: "I am an intelligent boy" doesn't match the meaning of the original sentence, so it should be corrected.)
This girl is very clever.
→ This is a very clever girl. (Correct)
He spends fifteen minutes having breakfast every day.
→ It takes him fifteen minutes to have breakfast every day. (Your suggested sentence "It took him ten minutes..." changes the time and tense. The correct version keeps the time and tense consistent.)
The house is very expensive. I can’t buy it.
→ The house is so expensive that I can’t buy it. (Correct)
John’s house is small.
→ John has a small house. (Correct)
Where are they from?
→ Where do they come from? (Correct)
How much is this cap?
→ How much is it? (Correct, but adding "this cap" clarifies what is being asked about: "How much is this cap?")
She’s older than her sister.
→ Her sister is younger than she is. (The last "she" in your version should be clarified as "she is.")
Do you cycle to school?
→ Do you get to school by bike? (Correct)
The bread is very dry. She can’t cut it.
→ It is such dry bread that she can’t cut it. (Correct)
The water is very cold. They can’t swim in it.
→ The water is not warm enough for them to swim in. (Correct)
The last time he drank coffee was two days ago.
→ It has been two days since he last drank coffee. (Your version "It was two days..." should be corrected to "It has been two days...")
The last time I read this book was 5 days ago.
→ It has been five days since I last read this book. (Similar correction: change "It is five days..." to "It has been five days...")