91Ó°ÊÓ

Due to the change in prices, the relative price of old textbooks to new textbooks decreased from 0.625 to 0.596, as computed below:

$$\begin{gathered} \mathrm{Relative}\mathrm{price}\mathrm{at}\mathrm{old}\mathrm{prices} \\ \frac{50}{80}=0.625 \\ \mathrm{Relative}\mathrm{price}\mathrm{at}\mathrm{new}\mathrm{prices} \\ \frac{52.5}{88}=0.596 \\ \mathrm{Since},\mathrm{for}10\%\mathrm{rise},\mathrm{the}\mathrm{price}\mathrm{of}\mathrm{new}\mathrm{textbook}\mathrm{will}\mathrm{be} \\ 80+\frac{10}{100}×80=\$88 \\ \mathrm{for}5\%\mathrm{rise},\mathrm{the}\mathrm{price}\mathrm{of}\mathrm{old}\mathrm{textbook}\mathrm{will}\mathrm{be} \\ 50+\frac{5}{100}×50=\$52.5 \end{gathered}$$

Antonio will react to the relative price decrease in the following two ways.

1. If new and old texts are not substitutes, Antonio will be just as well off when the price of new texts rises, and his father gives him $40.

U₁ is the indifference curve when goods are not substitutes.

L₁and L₂ are the initial and final price line/budget lines.

Suppose Antonio chooses to buy more old textbooks due to a relative price decrease. His indifference curve shifts upward, showing his increased consumption of old textbooks (if the old textbooks are substitutes for the new ones) when his income increases.

Antonio moves from indifference curve U₁ to U₂, shown in the figure above.

If Antonio chooses to buy a greater number of old textbooks in response to the relative price decrease (given the extra $40) and moves to a higher indifference curve.

A shift from a lower indifference curve to a higher indifference curve means that Antonio’s welfare has improved, and he is better off due to the price change.