[tex]\qquad \textit{Amount for Exponential Decay} \\\\ A=P(1 - r)^t\qquad \begin{cases} A=\textit{current amount}\dotfill &\stackrel{half~empty}{25~\hfill }\\ P=\textit{initial amount}\dotfill &50\\ r=rate\to 4\%\to \frac{4}{100}\dotfill &0.04\\ t=\textit{elapsed time}\\ \end{cases}[/tex]
[tex]25=50(1-0.04)^t\implies \cfrac{25}{50}=(1-0.04)^t\implies \cfrac{1}{2}=0.96^t \\\\\\ \log\left( \cfrac{1}{2} \right)=\log(0.96^t)\implies \log\left( \cfrac{1}{2} \right)=t\log(0.96) \\\\\\ \cfrac{\log\left( \frac{1}{2} \right)}{\log(0.96)}=t\implies 16.9797\approx t\implies \stackrel{hr}{17}\approx t[/tex]
