Write a system of equations to describe the situation below, solve using elimination, and fill in the blanks.

Two coworkers picked up some writing instruments at the office supply store. Sarah selected 9 boxes of pencils and 4 boxes of ballpoint pens, paying $25. Next, Heather spent $40 on 4 boxes of pencils and 9 boxes of ballpoint pens. How much does a box of each cost?
A box of pencils costs $, and a box of pens costs $.