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Chain Rule, Log Differentiation & Rate of Change · Question 77 of 130

After how many years will the population be growing at 2,000 people per year?

Question 77

HardOriginal question

A town's population tt years from now is modelled by P(t)=50,000 e0.02tP(t) = 50{,}000\,e^{0.02t}.

After how many years will the population be growing at 2,000 people per year?

Options for question 77

Show the answer and explanation
CorrectNot quiteAnswer B

Correct answer: B

Explanation

  1. P′(t)=1000 e0.02t=2000P'(t) = 1000\,e^{0.02t} = 2000 gives e0.02t=2e^{0.02t} = 2.

  2. 0.02t=log⁡20.02t = \log 2, so t=50log⁡2≈34.7t = 50\log 2 \approx 34.7 years.

Worth noting

log⁡250\frac{\log 2}{50} multiplies by 0.02 where it should divide: t=log⁡20.02t = \frac{\log 2}{0.02}.

Concept
When a rate reaches a given value
Typical time
150 s
Your time
—
The rule this testsExponential and logarithmic functions
Practise this chapter →

Ten questions from Chain Rule, Log Differentiation & Rate of Change, at your own pace.

Practise this chapter →

Ten questions from Chain Rule, Log Differentiation & Rate of Change, at your own pace.