For question number $12$: Solve.

The function $A=A_0{e}^{-0\mathrm{.}00693x}$ models the amount in pounds of a particular radioactive material stored in a concrete vault, where x is the number of years since the material was put into the vault, $Ao$ is the original amount of the material, and $A$ is the amount of the material at time $t.$ If $900$ pounds of the material are placed in the vault, how much time will need to pass for only $731$ pounds to remain?

$60$ years

$35$ years

$40$ years

$30$ years