There are two consumers of flu vaccine, a public good. Dian's benefit from flu vaccine is given by MBD = 100 - Q, where Q is the quantity of vaccine. Laila's benefit is given by MBL = 60 - Q.

a) Derive the total marginal benefit, MBt ( 5 marks )

b) Suppose that flu vaccine can be provided at a marginal cost of MC=2Q. Find the optimal level of flu vaccine.( 5 marks )

c) How much benefit do Dian and Laila enjoy at the optimal level of flu vaccine? (Assume Dian and Laila do not have to bear any of the cost personally, but that vaccine is provided by the government at no direct cost to the recipient).( 10 marks )