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3. Alvin's demand for bottled water is given by the equation QA = 8 - .5P. Betty's demand function is QB = 6 - P. (1 mark) a. Calculate Alvin and Betty's marginal and total willingness to pay for four bottles of water and illustrate graphically. b. Compute the aggregate demand for bottled water, assuming Alvin and Betty are the only consumers. c. Derive the aggregate demand curve if there were five people with Alvin's demand curve and 10 people like Betty