IN the right panel of Figure 12-1, the MPBa gives the marginal private benefit individual A receives for each additional hour per week devoted to tending his or her yard (i.e., cutting grass, planting flowers, etc.), thereby increasing the value of individual A’ s home and providing relaxation. The private marginal cost incurred by individual A for tending the yard (the depreciate ion on the lawn mowers, the cost of seeds, etc.) is given by the MPCa curve. Therefore, the best number of hours to spend tending the yard is 6 hours per week and is given by point Ea, A also confers a private benefit to individual B (i.e., it also increases the value of individual B’s home ). If the marginal private conferred on individual B is given by the MPBb curve, the total marginal social benefit is then given by the MSB curve, which is the vertical summation of the MPBa and the MPBb curves. The best number of 6 hours per week spent working in the yard by individual A is not socially efficient, however, because MSB=$12, while MPCA=MSC=$8. From society ‘s point of view, the most efficient number of hours for individual A to work in his or her yard is 10 hours per week and is given by point Es, at which MPCa = MSC= MSB= $9. In cases such as these where the private costs and benefits do not coincide with the social costs and benefits, government intervention in the economy is justified in order to induce the production and/or the consumption of the product or service until the marginal social cost is equal to the marginal social benefit (i.e., until MSC=MSB ).