\[\frac{4^{- 3} \cdot 4^{- 5}}{2^{- 20}} = \frac{4^{- 8}}{2^{- 20}} = \frac{\left( 2^{2} \right)^{- 8}}{2^{- 20}} =\]
\[= \frac{2^{- 16}}{2^{- 20}} = 2^{4} = 16.\]