$\frac{ 4.2 \times {10}^{2} }{ 2.1 \times {10}^{3} }$
Separate the fraction into $2$ fractions$\frac{ 4.2 }{ 2.1 } \times \frac{ {10}^{2} }{ {10}^{3} }$
Divide the numbers$2 \times \frac{ {10}^{2} }{ {10}^{3} }$
Divide the terms with the same base by subtracting their exponents$2 \times {10}^{2-3}$
Calculate the difference$\begin{align*}&2 \times {10}^{-1} \\&0.2\end{align*}$