3. In economics, revenue R is defined as the amount of money derived from the sale of a product and is equal to the selling price p of the product times the number x of units actually sold. That is, R = xp. The Law of Demand states that p and x are related: as p increases, the number x of units sold decreases. Suppose that p and x are related by the following equation: p(x)= -0.1x+20, 0 <= x <= 200 . Express the revenue R as a function of x and find what number of units must be sold to maximize the revenue. What is the maximum revenue?