53. Average cost. Preston's Leatherworks finds that the cost, in dollars, of producing x belts is given by C(x) = 750 + 34x - 0.068x-. Find the rate at which average cost is changing when 175 belts have been produced. 54. Average cost. Tongue-Tied Sauces, Inc., finds that the cost, in dollars, of producing x bottles of barbecue sauce is given by C(x) = 375 + 0.75x /*. Find the rate at which average cost is changing when 81 bottles of barbe- cue sauce have been produced