\[16^{- 5}\ :8^{- 6} = \left( 2^{4} \right)^{- 5}\ :\left( 2^{3} \right)^{- 6} =\]
\[= 2^{- 20}\ :2^{- 18} = 2^{- 2} = \frac{1}{4}\]