\[\boxed{\text{5.\ 10}}\]
\[\textbf{а)} - 3 - x + x + 7,5 = 4,5\ \]
\[\textbf{б)}\ 2,7 + c + ( - c - 48) =\]
\[= 2,7 + c - c - 48 = - 45,3\]
\[\textbf{в)}\ n + 21 + ( - 21 + m) =\]
\[= n + 21 - 21 + m = n + m\]
\[\textbf{г)}\ x + z + (y - z) =\]
\[= x + z + y - z = x + y\]
\[\textbf{д)} - z + m + ( - z - m) =\]
\[= - z + m - z - m = - 2z\]
\[\textbf{е)}\ a - c + (c - a) =\]
\[= a - c + c - a = 0\ \]