This question was the most unpopular among the candidates. The performance of the few candidates who attempted it was below average. Candidates were expected to distinguish between competitive demand and joint demand, and explain using appropriate diagrams how the equilibrium price and quantity of commodity R in the market will be affected by the factors stated above in the (a) and (b) parts of the question respectively. Most of the candidates that attempted this question were able to distinguish between competitive demand and joint demand but performed poorly in drawing and using diagrams to explain how the stated factors above will affect the equilibrium price and quantity of commodity R in the market in the (a) and (b) parts of the question respectively, hence their performance in this question was poor.
The candidates were expected to answer thus:

(a)  Competitive demand is the demand for commodities which can be used interchangeably. An increase in the demand for one will lead to a decrease in the demand for its substitute. Joint demand on the other hand is the demand for commodities which are used together to satisfy a want. An increase in the demand for one will lead to an increase in the demand for the other.

(b)(i)    From the diagram above (figure 1), an increase in the price of the complement of commodity R from P1 to P2 has resulted in a fall in quantity demanded from Q1 to Q2. Demand for commodity R shifted from D1D1 to D2D2 and the equilibrium price fell from P1 to P2 and quantity from Q1 to Q2.

(ii)  An increase in the price of a substitute of commodity R will result in a fall of its quantity demanded from Q1 to Q2 (in figure 3). As a result of this, demand for commodity R will increase, shown by the demand curve shifting from D1D1 to D2D2 in figure 4. The   equilibrium price will increase from P1 to P2 and equilibrium quantity from Q1 to Q2.

(iii) The imposition of an indirect tax on commodity R will result in a shift of the supply curve from S1S1 to S2S2 in figure 5. With demand unchanged, the equilibrium price will increase from P1 to P2 but the equilibrium quantity will fall from Q1 to Q2.