well, there are 10 years in 1 decade, so 8.7 decades will just be 8.7*10 = 87 years, so
[tex]\qquad \textit{Amount for Exponential Decay} \\\\ A=P(1 - r)^t\qquad \begin{cases} A=\textit{current amount}\\ P=\textit{initial amount}\dotfill &230\\ r=rate\to 7\%\to \frac{7}{100}\dotfill &0.07\\ t=years\dotfill &87\\ \end{cases} \\\\\\ A=230(1 - 0.07)^{87}\implies A=230(0.93)^{87}\implies A\approx 0.42[/tex]
