The value of an account that is being compounded continuously is given by the formula $A=P e^{r \prime}$, where $P$ is the principal, $r$ is the annual interest rate, and $t$ is the time in years. Approximately how long will it take for the amount of money to double if the interest rate is $2.4 \%$ ?
11.0 years
12.9 years
20.0 years
28.9 years