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?