A Public GoodPanel (a) shows Ted’s marginal private benefit curve, MPBT, and panel (b) shows Alice’s marginal private benefit curve, MPBA. Panel (c) shows the marginal social benefit of the public good, equal to the sum of the marginal private benefits to all consumers (in this case, Ted and Alice). The marginal social benefit curve, MSB, is the vertical sum of the marginal private benefit curves MPBT and MPBA. At a constant marginal social cost of $6, there should be 5 street cleanings per month, because the marginal social benefit of going from 4 to 5 cleanings is $8 ($3 for Ted plus $5 for Alice), but the marginal social benefit of going from 5 to 6 cleanings is only $2.