Mrs. bell's class is selling hobbs middle jaguar t-shirts to raise money for a trip. they typically sell 400 t-shirts a year for $10 per shirt. mrs bell knows that for every $1 increase in price, they will sell 20 less t-shirts. what is the revenue function r(x), where x represents the $1 price increases and revenue is equal the number of t-shirts sold times the cost of each shirt