Each year, 1 thousand people benefit from the Potomac River's recreation in Western Maryland. It is estimated that each beneficiary has the same demand function for the river recreation as: P=120-2Q where Q is measured in miles of the river preservation for recreation, P-price per mile of the river preserved or individual willingness to pay for recreation. Given that the marginal cost of river preservation is estimated at $20,000 per mile per year, find:

What is aggregate demand for 1000 people? [Apply vertical summation for public goods].

what would be the first step to complete this problem?