Answer
about 8.15 years
Work Step by Step
If a principal $P$ is invested in an account paying an annual interest rate $r$,
and the interest is compounded continuously, then the amount after t years is
$A(t)=Pe^{rt}$
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Solve for t after inserting given values
$2000=1000e^{0.085t} \qquad$ ... $/\div 1000$
$2=e^{0.085t} \qquad$ ... apply $\ln$() to both sides
$\ln 2=0.085t \qquad$ ... $/\div=0.085$
$t=\displaystyle \frac{\ln 2}{0.085}\approx 8.15$ years.
The investment will double in about 8.15 years.
