JAMB 2019 Economics · Question 35

Let the quantity demanded in units of a particular commodity be represented as Qd = 80 - 2P, find the quantity demanded when P = ₦3.

Options

A.70
B.47
C.74
D.40

Answer: C. 74

Explanation written with AI and checked against the answer key

Analyze the question: You're given a linear demand function, Qd = 80 - 2P, and asked to find the quantity demanded when the price P = ₦3. Concept: A demand function shows the relationship between the price of a commodity and the quantity consumers want to buy. To find Qd at a given price, substitute the price into the equation. Solution: Qd = 80 - 2P Substitute P = 3: Qd = 80 - 2(3) Qd = 80 - 6 Qd = 74 units Answer: The quantity demanded is 74 units, which is option C. Tip: In substitution questions, do the multiplication first (2 × 3 = 6) before subtracting. A common mistake is to compute (80 - 2) × 3 or to subtract 2 and 3 first. Remember BODMAS: multiplication comes before subtraction. Also, a quick sanity check helps: since the demand curve slopes downward, a small price of ₦3 should give a quantity just below 80, and 74 fits that.

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