Here are the corrections for the sentences provided:

11. London is an expensive city. *(Correct as is.)*

12. He is **the** most intelligent student in our class.

13. **Some** students decided to leave the party early because it was so boring.

14. My father's office **is** on the second floor.

15. Peter earned **a lot of** money last year.

16. Your new car is **cheaper** than John's.

17. Why do Suzie and Guy feel tired?

18. He left college when he **was** 17.

19. Joe always puts **a lot of** salt on his food.

20. We'll have to hurry. We haven't got **much** time.

21. I don't know **many** people in this town.

22. I use the phone **a lot** at work.

23. There wasn't **much** traffic this morning.

24. You need more money to travel around the world. *(Correct as is.)*