## Precalculus (6th Edition) Blitzer

$\approx 6.9\%$
Re-arrange as: $2=e^{10 r}$ We need to take $\ln$ of each side of the equation. $\ln 2 =\ln e^{10 r}$ Use the logarithmic property $\ln e^a =a$ with $a=10r$ to obtain $\ln 2= 10r \implies r = \dfrac{\ln 2}{10} \approx 0.069$ or, $\approx 6.9\%$ We can see that the money will be doubled in 10 years with an interest rate of $\approx 6.9\%$ .