11. He is the tallest in his family.
→Noone in his family  is as tall as him.
12. This exercise is the most difficult in this book.
→ No other exercise in this book.......is more difficult than this.....
13. No other city in Viet Nam is more beautiful than Da Lat.
→ Da Lat is  the most beautiful city in Viet Nam.
14. Lan is more beautiful than all girls in her class.
→ Lan is the most beautifull girl in her class.
15. He couldn't find a cheaper computer.
→ That was the cheapest computer he could find