Craig is still consuming apples and bananas. His utility function is U(xA, X) = xA*X. We want to find his demand function for apples, xa(Pa,P,m), and his demand function for bananas, XB(PA,PB,m). His MRS is -XB / XA.
a) When the prices are Pa and P and Craig's income is m, the equation for Craig's budget line is Pa*xa + P*X = m. What is the slope of Craig's indifference curve at the bundle (xa,X)? What is the slope of Craig's budget line? Write the equation that needs to be satisfied for Craig's indifference curve to be tangent to his budget line at the point (xa,X).
b) You now have two equations, the budget equation and the tangency equation, that must be satisfied by the bundle demanded. Solve these two equations to find the optimal xa and X.
c) In general, the demand for both commodities will depend on the price of both commodities and on income. But for Craig's utility function, the demand function for apples depends only on income and the price of apples. Similarly, the demand for bananas depends only on income and the price of bananas. Craig always spends the same fraction of his income on bananas. What fraction is this?