Two commodities X and Y are in joint supply when

Step 1: Analyze the question The question asks for the definition of joint supply. Think of it as: when you produce one good, you automatically produce another.

Step 2: Explain the concept Joint supply (also called joint products) happens when two or more goods come from the same production process, so a change in the supply of one directly changes the supply of the other. The goods are produced together, in some fixed proportion.

Nigerian examples:

• Beef and hides/skin (ponmo): rearing and slaughtering cattle gives both.
• Palm oil and palm kernel: processing palm fruit gives both.
• Petrol, diesel, kerosene: refining crude oil gives all of them.
• Cotton lint and cotton seed.

If more cattle are slaughtered for beef, the supply of hides also increases.

Step 3: Check each option

• A. X is a by-product of Y: Correct. If X is a by-product, it comes out of producing Y. Increasing Y automatically increases X, which is exactly joint supply.
• B. X and Y are produced by the same firm: Not enough. A firm can make bread and shoes separately, and those are not jointly supplied.
• C. Increase in the quantity of X leads to a decrease in Y: This describes competitive supply (e.g. a farmer using land for either yam or cassava), not joint supply.
• D. X and Y cannot be produced in the same process: This is the opposite of joint supply.

Step 4: Answer The answer is A.

Step 5: Exam tip Remember these three terms:

• Joint supply: the goods come together (beef and hides). Their supply moves in the same direction.
• Competitive supply: the goods compete for the same resources (yam vs cassava). Their supply moves in opposite directions.
• Complementary demand: goods used together (car and fuel). This is about demand, not supply.

If an option says "one is a by-product of the other", it is almost always joint supply.