Homer and Ned are neighbors. During the winter, it is impossible for a snowplow to clear the street in front of Ned's house without clearing the front of Homer's too. Ned's marginal benefit from snowplowing services is 40 4, where is the number of times the street is plowed per month. (Note: Z is a rate over time, so it need not be an integer. There is always a time dimension to any quantities that we discuss, so having fractional quantities is never a problem.) Homer's marginal benefit is 20 2. Barney, the Plow King, charges $18 each time he plows the street.