\[\boxed{\mathbf{28}\mathbf{.}}\]
\[1)\ \sqrt[6]{36^{3}} = \sqrt[6]{\left( 6^{2} \right)^{3}} = \sqrt[6]{6^{6}} = 6\]
\[2)\ \sqrt[12]{64^{2}} = \sqrt[12]{\left( 2^{6} \right)^{2}} =\]
\[= \sqrt[12]{2^{12}} = 2\]
\[3)\ \sqrt[4]{\left( \frac{1}{25} \right)^{2}} = \sqrt[{2 \bullet 2}]{\left( \frac{1}{25} \right)^{2 \bullet 1}} =\]
\[= \sqrt{\frac{1}{25}} = \frac{1}{5}\]
\[4)\ \sqrt[8]{225^{4}} = \sqrt[8]{\left( 15^{2} \right)^{4}} =\]
\[= \sqrt[8]{15^{8}} = 15\]