$a)8^{-3}\cdot {\left(\frac{1}{4}\right)}^{-3}={\left({\sout{8}}^2\cdot \frac{1}{{\sout{4}}^1}\right)}^{-3}=2^{-3}=\frac{1}{2^3}=\frac{1}{8}$

$b){20}^{-6}\cdot {0,5}^{-6}={\left(20\cdot 0,5\right)}^{-6}={10}^{-6}=\frac{1}{{10}^6}=\frac{1}{1000000}$

$c){\left(3\frac{1}{5}\right)}^{-2}\cdot {\left(1\frac{1}{4}\right)}^{-2}={\left(3\frac{1}{5}\cdot 1\frac{1}{4}\right)}^{-2}={\left(\frac{{\sout{16}}^4}{{\sout{5}}^1}\cdot \frac{{\sout{5}}^1}{{\sout{4}}^1}\right)}^{-2}$ $=4^{-2}=\frac{1}{4^2}=\frac{1}{16}$