79. Rate of change of cost. A fast-food restaurant can produce
a hamburger for $2.50. If the restaurant's daily sales are in-
creasing at the rate of 30 hamburgers per day, how fast is its
daily cost for hamburgers increasing?
80. Rate of change of cost. The fast-food restaurant in Problem
79 can produce an order of fries for $0.80. If the restaurant's
daily sales are increasing at the rate of 45 orders of fries per
day, how fast is its daily cost for fries increasing?