Petrol and Kerosene are jointly obtained from crude oil. If the supply of petrol increases, the

Step 1: Analyze the question Petrol and kerosene are obtained from the same source, crude oil. This is a classic case of joint supply.

Step 2: The concept Joint supply (also called joint products) means that two or more goods are produced together from the same raw material or process. Producing more of one automatically yields more of the other, so their supplies move in the same direction.

Common examples:

• Beef and hides (leather)
• Petrol and kerosene
• Mutton and wool

Step 3: Apply it

• If the supply of petrol increases, it means more crude oil is being refined.
• Refining more crude oil also produces more kerosene as a by-product.
• So the supply of kerosene will rise.

Step 4: Check the options

• A. Cost of crude oil production has increased. This is not a consequence of increased petrol supply, so it is wrong.
• B. Supply of kerosene will rise. This is correct and matches joint supply.
• C. Supply of kerosene will remain unchanged. This would apply only to unrelated goods, so it is wrong.
• D. Supply of kerosene will fall. This describes competitive supply (e.g., yam vs. maize on the same farmland), not joint supply, so it is wrong.

Answer: B

Exam tip: Ask yourself, "Do the goods come from the same source?"

• Same source gives joint supply, and the supplies move together.
• Same resources used for alternative products gives competitive supply, and the supplies move in opposite directions.