\[\boxed{\text{5.\ 115}}\]
\[\textbf{а)}\ 3a - (a + 4) = 3a - a - 4 =\]
\[= 2a - 4\]
\[\textbf{б)}\ a + 4 \cdot (5a - 2) =\]
\[= a + 20a - 8 = 21a - 8\]
\[\textbf{в)}\ n + 0,5 \cdot (4n - 10) =\]
\[= n + 2n - 5 = 3n - 5\]