\[\boxed{\mathbf{778}\mathbf{.}}\]
\[1)\log_{2}15 - \log_{2}\frac{15}{16} =\]
\[= \log_{2}\left( 15\ :\frac{15}{16} \right) =\]
\[= \log_{2}16 = \log_{2}2^{4} = 4\]
\[2)\log_{5}75 - \log_{5}3 = \log_{5}\frac{75}{3} =\]
\[= \log_{5}25 = \log_{5}5^{2} = 2\]
\[3)\log_{\frac{1}{3}}54 - \log_{\frac{1}{3}}2 = \log_{\frac{1}{3}}\frac{54}{2} =\]
\[= \log_{\frac{1}{3}}27 = \log_{\frac{1}{3}}3^{3} =\]
\[= \log_{\frac{1}{3}}\left( \frac{1}{3} \right)^{- 3} = - 3\]
\[4)\log_{8}\frac{1}{16} - \log_{8}32 =\]
\[= \log_{8}\left( \frac{1}{16}\ :32 \right) =\]
\[= \log_{8}\frac{1}{512} = \log_{8}8^{- 3} = - 3\]