Ryan is a software salesman. His base salary is $2500 , and he makes an additional $90 for every copy of History is Fun he sells. Let P represent his total pay (in dollars), and let N represent the number of copies of History is Fun he sells. Write an equation relating P to N . Then use this equation to find his total pay if he sells 21 copies of History is Fun.
equation:
total pay if 21 copies are sold?