Here are the sentences with "the" or "a" added where necessary:

1. You'll find **the** information you need at **the** top of page 15.
2. I'm fed up with doing **the** same thing every day.
3. Room 25 is on **the** second floor.
4. **The** Moon goes round **the** Earth every 27 days.
5. It was **a** very hot day. It was **the** hottest day of the year.