Answer
$\approx$ $0.132$ $Ω/s$
Work Step by Step
with $R_{1}$ = $80$ and $R_{2}$ = $80$
$\frac{1}{R}$ = $\frac{1}{R_{1}}+\frac{1}{R_{2}}$
$\frac{1}{R}$ = $\frac{1}{80}+\frac{1}{100}$
$R$ = $\frac{400}{9}$
differentiating
$\frac{1}{R}$ = $\frac{1}{R_{1}}+\frac{1}{R_{2}}$
$-\frac{1}{R^{2}}\frac{dR}{dt}$ = $-\frac{1}{R_{1}^{2}}\frac{d{R_{1}}}{dt}-\frac{1}{R_{2}^{2}}\frac{d{R_{2}}}{dt}$
$\frac{dR}{dt}$ = $R^{2}[\frac{1}{R_{1}^{2}}\frac{d{R_{1}}}{dt}
+\frac{1}{R_{2}^{2}}\frac{d{R_{2}}}{dt}]$
$\frac{dR}{dt}$ = $\frac{400^{2}}{9^{2}}[\frac{1}{80^{2}}(0.3)+\frac{1}{100^{2}}(0.2)]$ $\approx$ $0.132$ $Ω/s$
