Essentials of Economics 7th Edition by Gregory Mankiw

Exercise 17

Suppose the price elasticity of demand for heating oil is 0.2 in the short run and 0.7 in the long run.
a. If the price of heating oil rises from $1.80 to $2.20 per gallon, what happens to the quantity of heating oil demanded in the short run? In the long run? (Use the midpoint method in your calculations.)
b. Why might this elasticity depend on the time horizon?