\[\boxed{\text{6.3}\text{3}\text{9.}}\]
\[\textbf{а)}\ \frac{1}{8}\ :0,4 = 0,125\ :0,4 =\]
\[= 1,25\ :4 = 0,3125\]
\[\textbf{б)}\ \left( 5,75 - 3\frac{1}{4} \right)\ :0,8 =\]
\[= (5,75 - 3,25)\ :0,8 =\]
\[= 2,5\ :0,8 = 25\ :8 = 3,125\ \]
\[\textbf{в)}\ (1 - 0,5556)\ :\frac{11}{40} =\]
\[= 0,4444\ :0,275 =\]
\[= 444,4\ :275 = 1,616\ \]
\[\textbf{г)}\ 12,525\ :\left( \frac{4}{5} + 0,7 \right) =\]
\[= 12,525\ :(0,8 + 0,7) =\]
\[= 12,525\ :1,5 =\]
\[= 125,25\ :15 = 8,35\ \]