You give a salesperson $20 for a purchase and receive change. which function would you use to find the amount of change from a $4.50 purchase? what is the correct change? a. a(c) = c - 20; $5.50 c. a(c) = c - 20; $15.50 b. a(c) = 20 - c; $5.50 d. a(c) = 20 - c; $15.50