The following equations represent supply and demand functions for electricity in a certain city:
P = 100 + 0.3 Q (supply function without external costs)
P = 120 + 0.3 Q (supply function with external costs)
P = 250 - 0.2 Q (demand function)
Compute the net surplus (= PS+CS - total environmental external costs) at private market equilibrium and social efficient market equilibrium.