(30^-1+60^-1+80^-1)^-1

(30^-1+60^-1+80^-1)^-1

$$\small{\text{
$
\begin{array}{rcl}
&&\left(
30^{-1}+60^{-1}+80^{-1}
\right)^{-1}\\\\
&=& \left(
\dfrac{1}{30}+\dfrac{1}{60}+\dfrac{1}{80}
\right)^{-1}\\\\
&=& \left(
\dfrac{60\cdot 80+ 30\cdot 80+30\cdot 60}{30\cdot 60 \cdot 80}
\right)^{-1}\\\\
&=& \dfrac{30\cdot 60 \cdot 80} {60\cdot 80+ 30\cdot 80+30\cdot 60}\\\\
&=& \dfrac{144000} {9000}\\\\
&=& 16
\end{array}
$}}$$