\[\boxed{\mathbf{6.102}}\]
\[\textbf{а)}\ \frac{1}{5} \cdot (5 - 15a) =\]
\[= \frac{1}{5} \cdot 5 - \frac{1}{5} \cdot 15a = 1 - 3a\]
\[\textbf{б)}\ \left( \frac{5}{8} - n \right) \cdot \frac{4}{5} = \frac{5}{8} \cdot \frac{4}{5} - n \cdot \frac{4}{5} =\]
\[= \frac{1}{2} - \frac{4}{5}n\]