9. (15 points) 12 children jointly buy a set of books, and the cost of buying the books is shared equally among them. Since 2 of the children did not bring money when purchasing, the remaining 10 children each paid an extra 10 yuan. How much does it cost to buy the set of books in total?
【Analysis】First, find out how much more 10 children spent based on the difference in quantity, then evenly distribute it to 2 children, and the solution can be found.

【Solution】Solution: According to the problem;
The extra amount paid by 10 children is the amount for 2 children, i.e., $10 \times 10=100$ yuan, so each child should pay 100 $\div 2=50$ yuan.

The total amount 12 children should pay is: $12 \times 50=600$ yuan.
Answer: The total cost to purchase this set of books is 600 yuan.