Inverse demand functions.

To put it blunt, my teacher for Environmental Economics doesn't teach any math but put math on our next test.

Any help with the two problems and an explanation on how to do it would be great.

1.) There are 100 consumers of a public good. Each consumer has an identical inverse demand function given by P=10-Q. where Q measures the number of units of the good and P is the price in dollars.

a.) If the marginal cost of the public good is $500 per unit, what is the economically efficient level of production of the public good?

b.) How large are the net benefits?

I don't even know what this has to do with environmental economics, but any help would be great.