Question

A bacterial culture starts with 400 bacteria and grows at a rate proportional to its size. After 2 hours there will be 800 bacteria.

1) Express the population, P(t) after t hours as a function of t
2) Using your equation from above, what will the population be after 8 hours?
3) Using your equation from above, after how many hours will the population reach 2270 bacteria?

Answer 1

1. P(t) = 400×2^(t/2)
2. 6400
3. 5.0

Explanation:

The wording "a rate proportional to its size" is an indication that growth (or decay) is exponential.

1) For many problems involving exponential growth, I like to use the numbers given in the problem statement as follows.

population = (initial population) × (growth factor)^(t/(growth period))

where the "growth period" is the period of time in which the population is multiplied by the "growth factor".

Here, we're given ...

(initial population) = 400

(growth factor) = 800/400 = 2

(growth period) = 2 . . . . hours

So, our population function can be written as ...

P(t) = 400×2^(t/2)

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2) Putting t=8 into the formula, we get ...

P(8) = 400×2^(8/2) = 400×16 = 6400

After 8 hours, the population will be 6400.

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3) Fill in the given number and solve for t.

2270 = 400×2^(t/2)

2270/400 = 2^(t/2)

Taking logs, we have ...

log(227/40) = (t/2)log(2)

t = 2×log(227/40)/log(2) ≈ 5.009

After 5.0 hours, the population will reach 2270.