\[\frac{a^{23} \cdot \left( b^{5} \right)^{4}}{(a \cdot b)^{20}} = \frac{a^{23}b^{20}}{a^{20}b^{20}} = a^{3};\ \]
\[a = 2:\]
\[a^{3} = 2^{3} = 8.\]