Set 2: Exponential Functions (Intermediate)

Choice A is correct. Choice A is the correct answer. Apply the half-life concept. 1. Half-lives: 10 years ÷ 5 years = 2 half-lives. 2. After 1 st half-life (5 years): $80\times 0.5 = 40$ grams. 3. After 2 nd half-life (10 years): $40\times 0.5 = 20$ grams. 4. Formula: $A(t) = 80(0.5)^{t/5} = 80(0.5)^2 = 20$. Strategic Tip: Number of half-lives = elapsed time ÷ half-life period. Choice B is incorrect because this is after only 1 half-life (5 years). Choice C is incorrect because this would be after 3 half-lives (15 years). Choice D is incorrect because this would be after 4 half-lives (20 years).