Alexander studies away from home. While at school, he spends all his income on air travel and economics textbooks. In a diagram with air-travel as the x-axis good, as pair increases while ptext remains unchanged, Alexander's price consumption path is horizontal.
a. How does Alexander's expenditure on air travel respond to changes in pair? Show that his demand for air travel is inversely proportional to pair.
b. What is the price elasticity of the demand for air travel?
c. Are textbooks complements or substitutes for air travel?