Market for steel is competitive. Demand for steel is given by Q = 60 -P/2 (i.e.marginal benefit is MB = 120 - 2Q). Private marginal cost of steel production is MC= Q. Marginal damage caused to the society from steel production is MD = 2Q. What is the social cost (i.e. the deadweight loss) of steel production in thecompetitive equilibrium?

The asnwer is 640, how do I derive that