Set 13: Exponential Functions

Choice A is correct. Choice A is the correct answer. Work backwards from the remaining percentage. 1. Remaining: 12.5% = $\frac{1}{8} = (\frac{1}{2})^3$. 2. Half-lives: 3 half-lives have occurred. 3. Half-life period: $\frac{15}{3} = 5$ days. 4. Verification: After 15 days = 3 periods: $(0.5)^3 = 0.125 = 12.5\%$ ✓ Strategic Tip: 12.5% = $\frac{1}{8}$ means 3 halvings have occurred. Choice B is incorrect because this would give 2 half-lives in 15 days. Choice C is incorrect because this would mean only 1.5 half-lives. Choice D is incorrect because this would mean 4 half-lives.