Answer :
Given that for every $1 increase in the price of the t-shirts, they will sell 20 less shirts. The revenue function R(x) is giiven by R(x) = (400 - 20x)(10 + x) = 4000 + 200x - 20x^2. Therefore, the revenue function representing the revenue Mrs Bell's class earns from selling Hobbs Middle Jaquar t-shirts is given by R(x) = 4000 + 200x - 20x^2 where x represent the $1 price increases and revenue is equal to the number of t-shirts sold times the cost of each shirt.