In the left panel of Figure 12-1, MPBa refers to the marginal private benefit that individual A receives for each additional hour of making furniture at home in the evening after a regular full-time day job. If individual A is willing to work only from the best-paying customer (i.e.,$12). If individual A is willing to work for more hours per evening, he or she will have to accept work from customers who are willing to pay less and less, as indicated by the MPBa curve. The left panel of Figure 12-1 shows also the MPCa, or the marginal private cost, that individual A incurs in working each additional hour per evening. The best number of hours that individual A is then willing to work per evening is 4 and is given by point Ea, at which the MPBa and MPCa curves intersect.