6. (4 points) A store has been selling Hagen-Dazs bar at the price of $20 per box and, at this price, the store can sell 100 boxes per day. A customer survey indicates that for each $1 decrease in price per box, sales will increase by 2 boxes. (i) Find the demand function (or the price function) p(x), where x is the number of boxes sold per day. (ii) Find the marginal revenue at the sale of 50 boxes