Multiple Choice

An economics student is solving for an individual's optimal choice between consumption (c) and free time (t). The individual's willingness to trade is MRS = 40 / t and the technical possibility of trading is MRT = 4 / √(20 - t). The student's work is presented below:

Step 1: Set MRS = MRT → 40 / t = 4 / √(20 - t) Step 2: Simplify the equation → 10 / t = 1 / √(20 - t), which rearranges to 10 * √(20 - t) = t Step 3: Square both sides → 100 * (20 - t) = t^2 Step 4: Expand and rearrange into a quadratic equation → 2000 - 100t = t^2, which becomes t^2 + 100t - 2000 = 0 Step 5: Factor the quadratic equation → (t - 20)(t + 100) = 0 Step 6: Conclude that the solutions are t = 20 or t = -100. Since time cannot be negative or equal to 20 (which would make the MRT undefined), the student concludes there is no valid solution based on their work.

Analyze the student's work. Which statement best identifies the primary flaw in their reasoning?

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Updated 2025-07-29

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