Answer:
- P(t) = 400×2^(t/2)
- 6400
- 5.0
Step-by-step 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.
