Carmen is a software saleswoman. Her monthly salary is 
 $1800
 plus an additional 
 $80
 for every copy of Math is Fun she sells.
 Let 
 S
 represent her total salary this month (in dollars), and let 
 N
 represent the number of copies of Math is Fun she sells. Write an equation relating 
 S
 to 
 N
 , and then graph your equation using the axes below.