An initial investment of $4000 doubles in value in 6.8 years. Assuming continuous compounding, what was the interest rate?
I would use the model
$\displaystyle A=Pe^{rt}$
where A = annuities, P = principal amount invested, r = rate, t = time in years
Therefore
$\displaystyle A=4000e^{rt}$
as the amount doubled over 6.8 years, using (t,A) = (6.8,8000) gives
$\displaystyle 8000=4000e^{6.8\times r}$
Can you solve for r?
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