If you read 50 pages of a book every day, you can’t finish it in 8 days, and you have more than 9 days; if you read 60 pages a day, you can’t finish it in 7 days, and you have more than 8 days; if you read 7n pages a day, you can read it in exactly n days End (n is a natural number), how many pages are there in this book.
一本书，如果每天读50页，8天读不完，9天又有余；如果每天读60页，7天读不完，8天又有余；如果每天读7n页，又恰好用n天读完(n是自然数)，这本书共多少页．
<result>448</result>