Correct the errors. 1. is nick and his friend late? 2. these is my books. 3. where is the keys? 4. mathematics are difficult 5. my friend and i am in my room 6. are your sister twenty? 7. why is not you at school?

1. Are Nick and his friend late?
2. These are my books.
3. Where are the keys?
4. Mathematics is difficult.
5. My friend and I are in my room.
6. Is your sister twenty?
7. Why are not you at school?