[tex]\qquad \textit{Amount for Exponential Decay} \\\\ A=P(1 - r)^t\qquad \begin{cases} A=\textit{current amount}\dotfill &\$12000\\ P=\textit{initial amount}\dotfill &\$30000\\ r=rate\to 25\%\to \frac{25}{100}\dotfill &0.25\\ t=years\\ \end{cases}[/tex]
[tex]12000=30000(1 - 0.25)^{t}\implies \cfrac{12000}{30000}=(1 - 0.25)^{t}\implies \cfrac{2}{5}=0.75^t \\\\\\ \log\left( \cfrac{2}{5} \right)=\log(0.75^t)\implies \log\left( \cfrac{2}{5} \right)=t\log(0.75) \\\\\\ \cfrac{~~\log\left( \frac{2}{5} \right)~~}{\log(0.75)}=t\implies 3\approx t[/tex]