4. A group of management students are creating a business plan for their proposed shoe brand SoleSearching. The total production cost of r pairs of shoes is expected to be given by 1 1 C(x) = =r + 500r + 80000 pesos. The group intends to sell at p(r) = 1000 - -r pesos per pair. a. What will be the marginal profit when 100 pairs are sold? (2 points) b. At what production level will the average cost per pair be at a minimum? (3 points)