4
advanced-math

A fish population in a lake follows P(t)=80001+15e0.3tP(t) = \frac{8000}{1 + 15e^{-0.3t}} (in years). When does the population reach 4,000?

A

About 9 years

B

About 15 years

C

About 5 years

D

About 20 years

Correct Answer: A

Choice A is the correct answer. Solve for tt when P(t)=4000P(t) = 4000.

  1. Equation: 4000=80001+15e0.3t4000 = \frac{8000}{1 + 15e^{-0.3t}}.
  2. Multiply: 4000(1+15e0.3t)=80004000(1 + 15e^{-0.3t}) = 8000.
  3. Simplify: 1+15e0.3t=21 + 15e^{-0.3t} = 2.
  4. Isolate: 15e0.3t=115e^{-0.3t} = 1, so e0.3t=115e^{-0.3t} = \frac{1}{15}.
  5. Natural log: 0.3t=ln(115)=ln(15)2.708-0.3t = \ln(\frac{1}{15}) = -\ln(15) \approx -2.708.
  6. Solve: t=2.7080.39.039t = \frac{2.708}{0.3} \approx 9.03 \approx 9 years.

💡 Strategic Tip: For logistic functions, half the carrying capacity occurs when Aert=1Ae^{-rt} = 1.

Choice B is incorrect because this is too long. Choice C is incorrect because this is too short. Choice D is incorrect because this is way too long.