Chapter 5 - Section 5.2 - More Work with Exponents and Scientific Notation - Exercise Set - Page 268: 48

$\dfrac{25m^{}}{8n^{6}}$

Using laws of exponents, the given expression simplifies to \begin{align*} & \dfrac{2^{-3}m^{-4}n^{-5}}{5^{-2}m^{-5}n} \\\\&= \dfrac{5^{2}m^{-4-(-5)}n^{-5-1}}{2^{3}} \\\\&= \dfrac{25m^{}n^{-6}}{8} \\\\&= \dfrac{25m^{}}{8n^{6}} .\end{align*}