How is the efficient quantity of public goods determined?

What will be an ideal response?

The efficient quantity of a public good is determined in the same way as the efficient quantity of any good: by comparing marginal social cost and marginal social benefit, and producing where one equals the other. The marginal social cost of a public good is calculated similarly as the marginal social cost for a private good. It is based on the cost of producing one more unit of the good. The marginal social benefit, however, is calculated differently for a public good than for a private good. For a public good, the economy's marginal social benefit is determined by summing the individual marginal benefits at each quantity of the public good. Once the marginal social cost and marginal social benefits are known, the efficient quantity is equal to the quantity such that the marginal social cost equals the marginal social benefit.