$65\+20 e^{\ln \left(3\/4\right) t\/15}$

$\=98.6$

$20 e^{\ln \left(3\/4\right) t\/15}$

$\=33.6$

$e^{\ln \left(3\/4\right) t\/15}$

$\=1.68$

$\ln \left(3\/4\right) t\/15$

$\=\ln \left(1.68\right)$

$t$

$\=15 \frac{\ln \(1.68}{\ln \left(0.75\right)}$

$t$

$\doteq -27.05$

Define the function $u\left(t\right)\=65\+A e^{k t}$ 

$u\left(t\right)\=65\+A {\ⅇ}^{k t}$$\stackrel{\text{assign as function}}{\to }$$u$

Form and solve two algebraic equations for $A$ and $k$ 

$u\left(0\right)\=85\,u\left(15\right)\=80$

$65\+A\=85\,65\+A{\ⅇ}^{15k}\=80$

$\stackrel{\text{solve}}{\to }$

Obtain $u\left(t\right)$ and determine the time of death

$\genfrac{}{}{0ex}{}{u\left(t\right)}{\phantom{x\=a}}|\genfrac{}{}{0ex}{}{\phantom{\mathrm{f(x)}}}{}\=98.6$

$65\+20{\ⅇ}^{\frac{1}{15}\ln \left(\frac{3}{4}\right)t}\=98.6$

$\stackrel{\text{solve}}{\to }$

$\left\{t\=-27.05037139\right\}$