The graph shows the market demand and supply of coal from a strip mine. A graph shows quantity along the horizontal axis and price along the vertical axis. A demand curve with a negative slope starts from quantity 0, price 100 dollars and slopes down to quantity 45, price 9 dollars. The supply curve with a positive slope starts from quantity 0, price 10 dollars and slopes up to quantity 85, price 95 dollars. The coordinate points are as follows: quantity 30, price 40 dollars; quantity 20, price 60 dollars; quantity 10, price 80 dollars. All the data in the graph are approximate. If the marginal external cost of producing a ton of coal is $60 per ton, the socially optimal price of coal is $_____ per ton