\[\boxed{\mathbf{11}.}\]
\[1)\ \frac{12a^{2}b^{3}c^{5}}{27a^{4}b^{3}c^{2}} = \frac{4c^{3}}{9a^{2}}\]
\[2)\ \frac{a^{7}(a - b)^{2}\ }{a^{4}(a - b)^{3}} = \frac{a^{3}}{a - b}\]
\[3)\ \frac{2a + 6}{a^{2} - 9} = \frac{2 \cdot (a + 3)}{(a - 3)(a + 3)} =\]
\[= \frac{2}{a - 3}\]
\[4)\ \frac{(m - 2n)^{2}}{10n - 5m} = \frac{(2n - m)^{2}}{5 \cdot (2n - m)} =\]
\[= \frac{2n - m}{5}\]
\[5)\ \frac{a^{2} - 4}{a^{3} + 8} =\]
\[= \frac{(a - 2)(a + 2)}{(a + 2)\left( a^{2} - 2a + 4 \right)} =\]
\[= \frac{a - 2}{a^{2} - 2a + 4}\]
\[6)\ \frac{8a^{3} - 27}{9 - 4a^{2}} =\]
\[= \frac{(2a - 3)\left( 4a^{2} + 6a + 9 \right)}{(3 - 2a)(3 + 2a)} =\]
\[= - \frac{4a^{2} + 6a + 9}{2a + 3}\]