\[\boxed{\text{6.8.}}\]
\[\textbf{а)}\ 131\ :10 = \frac{131}{10} = 13,1;\ \]
\[\textbf{б)}\ 6\ 905\ :100 = \frac{6\ 905}{100} = 69,05;\ \]
\[\textbf{в)}\ 721\ :10\ 000 = \frac{721}{10\ 000} =\]
\[= 0,0721;\ \]
\[\textbf{г)}\ 66\ :1\ 000 = \frac{66}{1\ 000} = 0,066;\ \]
\[\textbf{д)}\ 931\ :1\ 000 = \frac{931}{1\ 000} = 0,931;\ \]
\[\textbf{е)}\ 7\ :100\ 000 = \frac{7}{100\ 000} =\]
\[= 0,00007.\ \ \ \]