\[\boxed{\mathbf{6.33}}\]
\[\textbf{а)}\ \frac{4,2}{0,7} = \frac{42}{7} = 6\]
\[\textbf{б)}\ \frac{0,85}{5} = \frac{8,5}{5} = 1,7\]
\[\textbf{в)}\ \frac{2,8 \cdot 8}{4} = 2,8 \cdot 2 = 5,6\]
\[\textbf{г)}\ \frac{25 \cdot 6}{30} = \frac{25}{5} = 5\]