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

The answer is R(x) = (10+x)(400-20x). 

If x is amount in dollars added to the original price, Then let "the cost of each shirt" be equal to the original price ($10) plus x. (10+x). And since for every unit increase in x, 20 units of shirts are subtracted from the original amount of shirt sold, Then let "the number of shirts sold" be equal to Twenty (20) units of shirts multiplied by the amount of dollars added to the original price (x), subtracted from The original amount of shirt sold (400). (400-20x). And since Revenue R(x) is equal to "the cost of each shirt" times "the number of shirts sold", therefore:
R(x)=(10+x)(400-20x)

or in quadratic form, R(x)=-20x^2+200x+4000.