$ANSWER:$
1. → This letter isn't clean enough for me to read.
→ This letter isn't too clean enough for me to read.
2. → That hat is too expensive for me to buy.
→ That hat is expensive enough for me to buy.
3. → This test is too easy for this boy to do.
→ This test is easy enough for this boy to do.
4. → These vegetables is fresh enough for us to eat.
→ These vegetables is too fresh for us to eat.
5. → This story is too interesting for you to read twice.
→ This story is interesting enough for you to read twice.