\[\boxed{\text{1203.\ }\text{еуроки}\text{-}\text{ответы}\text{\ }\text{на}\text{\ }\text{пятёрку}}\]
\[\textbf{а)}\frac{25^{m}}{5^{2m - 1}} = \frac{5^{2m}}{5^{2m - 1}} =\]
\[= 5^{2m - 2m + 1} = 5^{1} = 5\]
\[\textbf{б)}\ \frac{6^{m}}{2^{m - 1} \cdot 3^{m + 1}} =\]
\[= \frac{(2 \cdot 3)^{m}}{2^{m - 1} \cdot 3^{m + 1}} = \frac{2^{m} \cdot 3^{m}}{2^{m - 1} \cdot 3^{m + 1}} =\]
\[= 2^{m - m + 1} \cdot 3^{m - m - 1} =\]
\[= 2^{1} \cdot 3^{- 1} = \frac{2}{3}\]