Given
The demand equation is given
[tex]Q=2000P^{-k}[/tex]
here Q is liters purchased per year, P is the price in dollars per unit and k is a parameter =0.75.
Explanation
a. To determine the quantity of petrol demanded by the average motorist if the price is 1 dollar per liter.
[tex]\begin{gathered} Q=2000(1)^{-0.75} \\ Q=2000\text{ /year.} \end{gathered}[/tex]
b. To determine the change in quantity demanded.
It is given that the price of petrol increases from $1.0to $2.0 per litre on a sustained basis.
Then the equation becomes
[tex]\begin{gathered} Q_1=2000(1)^{-0.75} \\ Q_2=2000(2)^{-0.75} \end{gathered}[/tex]
The change in quantity demanded is
[tex]\Delta Q=\frac{Q_2-Q_1}{Q_1}[/tex]
Substitute the values.
[tex]\begin{gathered} \Delta Q=\frac{1189.2-2000}{2000} \\ \Delta Q=-\frac{810.8}{2000} \\ \Delta Q=-0.4054 \end{gathered}[/tex]Answer
a.The quantity of petrol demanded by the average motorist if the price is 1 dollar per liter is 2000 litres per year.
b. The change in quantity demanded is the fall in demand due to negative sign which is 0.4054.