P3.9 Demand Curves. The Eastern Shuttle, Inc., is a regional airline providing shuttle service between New York and Washington, DC. An analysis of the monthly demand for service has revealed the following demand relation where Q is quantity measured by the number of passengers per month, P is price ($), PQG is a regional price index for other consumer goods (1967 = 1.00), is an index of business activity, and S, a binary or dummy variable, equals 1 in summer months and 0 otherwise.