Giải thích các bước giải:

Do chữ số 0 không thể đứng đầu nên chữ số 9 phải đứng đầu. Mà các chữ số 0 và 9 xen kẽ nhau nên ta có cách sắp xếp như trên.

 Số đó là: 
909090
 

My dream school is a very beautiful school. It is a big and wide school. The school has many verdant trees and there There is also a stadium and sports ground for students to play there. What I like most about the school is that it is a great school with lots of modern equipment and technology. Moreover, the teachers at the school are very friendly and see them as knowledgeable people. It's all about the school I dreamed of the most