Answer
$m^{6}n^{18}$
Work Step by Step
Based on the product rule for exponents, we know that $a^{m}\times a^{n}=a^{m+n}$ (where $m$ and $n$ are positive integers and $a$ is a real number).
Therefore, $(m^{5}n^{10})(mn^{8})=m^{5+1}\times n^{10+8}=m^{6}\times n^{18}=m^{6}n^{18}$.