其它
On the first day of a $7-$day holiday, Judy reads $9$ pages of a book. On the second day, she reads $12$. Then each day later, she reads $3$ pages more than the day before. On the last day of the holiday, she reads the corresponding number of pages and exactly finishes reading the book. How many pages does the book have? 
Overseas Competition->Knowledge Point->Calculation Modules->Sequences and Number Tables->Arithmetic Sequences
Use the formula of arithmetic sequence to solve this problem.  On the last day, she reads $9+6\times3=27$ pages.  The book has $(9+27)\times7 \div2=126$ pages. 