Cho 2 đường thẳng a và b cắt nhau tại O. Trên a lấy 3 điểm A1, A2, A3 khác O. Trên b lấy 3 điểm B1, B2, B3 khác O. Hỏi có bao nhiêu tam giác được tạo thành từ 3 trong 7 điểm A1, A2, A3, B1, B2, B3 và O.

- Rewrite these sentences that it means the same as the sentence printed before it.

1. Does your class have twenty-five students?
    Are there..............................................................?
2. How much does a bowl of noodles cost?
    How much is......................................................?
3. The work is very hard and he can’t do it.
    This work is too...........................................
4. Mr. John has a daughter, Katy.
    Mr. John is ......................................................
5. What is the width of this river?
    How..........................................................?
6. My mother often drives to work.
    My mother often goes ....................................
7. What is the price of a cake and an orange?
    How...............................................................?
8. That book belongs to Nam.
   That is.....................................................