A publisher needs to send many books to a local book retailer and will send the books in a combination of small and large boxes. Each small box can hold 25 books and each large box can hold 40 books. A total of 7 boxes were sent which can hold 250 books altogether. Write a system of equations that could be used to determine the number of small boxes sent and the number of large boxes sent. Define the variables that you use to write the system.