WAEC Economics 2012 Paper

Concept:The demand function shows how quantity demanded ($Q_d$) changes with price ($P$).To find $Q_d$, substitute the given value of $P$ into the equation.Explanation:Given the demand function:$$Q_d = 20 - \frac{1}{2}P$$Here, $P$ is the price in dollars.We are asked to find $Q_d$ when $P = \$12$.Substitute $P = 12$ into the demand function:$$Q_d = 20 - \frac{1}{2}(12)$$First, calculate $\frac{1}{2} \times 12$:$$\frac{1}{2} \times 12 = 6$$Now subtract this value from 20:$$Q_d = 20 - 6$$$$Q_d = 14$$Thus, the quantity demanded is 14 units.Answer:$$Q_d = 14 \text{ units}$$So, the correct choice is Option D.