\[\boxed{\mathbf{6.107}}\]
\[\textbf{а)}\ 7y - 3,7 = 5y\]
\[7y - 5y = 3,7\]
\[2y = 3,7\]
\[y = 3,7\ :2\]
\[y = 1,85.\]
\[\textbf{б)}\ \frac{5}{6}n - 1 = \frac{1}{3}n\]
\[\frac{5}{6}n - \frac{2}{6}n = 1\]
\[\frac{3}{6}n = 1\]
\[\frac{1}{2}n = 1\]
\[n = 2.\]
\[\textbf{в)}\ 15,9a = 15,7a + 5\]
\[15,9a - 15,7a = 5\]
\[0,2a = 5\]
\[a = 5\ :0,2 = 50\ :2\]
\[a = 25.\ \]