The figures are invented. A diagram shows the market for a chemical whose production pollutes a river, with costs and benefits ($ per tonne) on the vertical axis and quantity (tonnes per week) on the horizontal axis. Demand (marginal social benefit) crosses supply (marginal private cost) at an output of 80 tonnes and a price of $50. Demand crosses the marginal social cost curve at an output of 60 tonnes and a price of $60. At an output of 80 tonnes marginal social cost is $75. (i) State the quantity by which the market overproduces. [1] (ii) Calculate the external cost of the 80th tonne. [1] (iii) Calculate the welfare loss at the market output. [2]

HL Paper 3-style calculation (standalone practice; not an IB question).