. Suppose that the price p (in dollars) and the weekly sales x (in thousands of units) of a commodity satisfy the demand equation 5p + x + xp = 120. How fast are the sales changing at a time when x = 4, p = 9 and the price is rising at the rate of $3 per week? (

A rectangular plot of land is to be bounded on one side by a river and on the other three sides by electric fencing. With 900ft. of fencing available, what is the largest area you can enclose?