The completed table is:
\[
\begin{tabular}{|rr|}
\hline $30 \, \text{ds} =$ & $\boxed{3 \, \text{s}}$ \\
\hline $\boxed{4 \, \text{km}} =$ & 4000 \, \text{m} \\
\hline $700 \, \text{cL} =$ & $\boxed{7 \, \text{L}}$ \\
\hline$\square \, \text{Mg} =$ & 5000000 \, \text{g} \\
\hline
\end{tabular}
\]