\[\boxed{\text{4.\ 365}}\]
\[m + n = n + m\]
\[\textbf{а)}\ m = 0,9;\ \ n = 2,4:\]
\(0,9 + 2,4 = 2,4 + 0,9\)глава
\[3,3 = 3,3\]
\[верно.\]
\[\textbf{б)}\ m = - 2\frac{1}{4};\ \ n = - 4\frac{1}{8}:\]
\[- 2\frac{1}{4} + \left( - 4\frac{1}{8} \right) = - 4\frac{1}{8} + \left( - 2\frac{1}{4} \right)\]
\[- \left( 2\frac{2}{8} + 4\frac{1}{8} \right) = - \left( 4\frac{1}{8} + 2\frac{2}{8} \right)\]
\[- 6\frac{3}{8} = - 6\frac{3}{8}\]
\[верно.\]