$\begin{align}  & {{\left( \frac{1}{0.06}\div \frac{1}{0.042} \right)}^{-1}}={{\left( \frac{1}{\tfrac{6}{100}}\div \frac{1}{\tfrac{42}{1000}} \right)}^{-1}}={{\left( \frac{100}{6}\div \frac{1000}{42} \right)}^{-1}} \\ & ={{\left( \frac{100}{6}\times \frac{42}{1000} \right)}^{-1}}={{\left( \frac{7}{10} \right)}^{-1}}=\frac{10}{7}\approx 1.43 \\\end{align}$