If commodities A & B are jointly demanded, what will be the effect of an increase in the price of A on the demand for commodity B?

Step 1: Analyze the question The question asks what happens to the demand for commodity B when the price of A rises, given that A and B are jointly demanded.

Step 2: Explain the concept Jointly demanded goods are complementary goods. They are used together to satisfy a single want. Examples include car and petrol, bread and butter, pen and ink, or tea and sugar.

For complements, the cross elasticity of demand is negative. A rise in the price of one good reduces the demand for the other.

Step 3: Work through the solution

1. The price of A increases.
2. By the law of demand, the quantity of A demanded falls.
3. Since B is used together with A, people now buy less B as well.
4. So the demand for B decreases. The demand curve for B shifts to the left.

For example, if the price of cars rises sharply, fewer people buy cars. The demand for petrol then falls.

Step 4: Check the other options

• B and E: These talk about the price of B changing. The question asks about the effect on demand for B, so they do not fit.
• C (remains constant): This would apply to unrelated goods, not complements.
• D (increase in demand): This would happen for substitutes, such as Gala and Biscuit, or rice and yam. It is not true for complements.

Correct answer: A. Decrease in demand for commodity B

Exam tip: Remember "Complements move opposite, substitutes move together."

• Price of A rises → demand for a complement falls.
• Price of A rises → demand for a substitute rises.

When you see "jointly demanded," think "complements."