\[\boxed{\mathbf{6.97}}\]
\[\textbf{а)}\ \ 21\ :56 = \frac{21}{56} = \frac{3}{8}\ часть.\]
\[\textbf{б)}\ 40\ :100 = \frac{40}{100} = \frac{4}{10} =\]
\[= 0,4\ часть.\]
\[\textbf{в)}\ 6\ :30 \cdot 100\% = 0,2 \cdot 100\% =\]
\[= 20\%\]
\[\textbf{г)}\ 82\ :574 \cdot 100\% =\]
\[= \frac{1}{7} \cdot 100\% = \frac{100}{7}\% = 14\frac{2}{7}\%\]