a. x = number of unit sold, y = income

y= 28.000, x<100

y= 28.000+0.05*x*7500, 100<x<200

y=28.000+0.05*(x-99)*7500+0.075*(x-199)*7500, x>200

b. x=338, use function 3

y=28.000+0.05*(x-99)*7500+0.075*(x-199)*7500

c. He must at least sold 20 unit (please check function 3)