When an increase in the price of a commodity lead to a fall In the demand for another, the demand for the two commodities are said to be

Analyze the question: The price of commodity X goes up, and the demand for a different commodity Y falls. We need the name for this relationship between the two goods.

Explain the concept:

• Joint (complementary) demand: Goods that are used together, such as car and petrol, or bread and butter. If the price of one rises, its quantity demanded falls, so less of the other is needed too. Demand for the partner good falls.
• Competitive demand (substitutes): Goods that can replace each other, such as Indomie and spaghetti. If the price of one rises, people switch to the other, so demand for the other rises.
• Composite demand: One commodity with several different uses, such as wood used for furniture, fuel and building.
• Derived demand: Demand for a good because it is needed to produce something else, such as demand for cement because people want buildings.

Step-by-step solution:

1. The price of commodity X rises.
2. Demand for commodity Y falls, so the two move in the same direction as each other's demand. This is the sign of goods used together.
3. Example: if the price of generators rises, fewer people buy them, so demand for petrol for generators falls.
4. This matches joint demand (complementary goods).

Check against the other options:

• A is wrong because competitive goods would see demand for Y rise.
• C is wrong because composite demand concerns one good with many uses.
• D is wrong because derived demand concerns inputs demanded for the sake of a final product.

Exam tip: Remember it this way: Complements = Joint = move together (the other good's demand falls). Substitutes = Competitive = move opposite (the other good's demand rises). In JAMB, look at the direction of the change in the second good's demand and you can decide quickly.