Answer
$4$
Work Step by Step
Substituting $s=4$ and $t=8$, the given expression, $
(st)^2\div(st^2)
,$ evaluates to
\begin{array}{l}\require{cancel}
(4\cdot8)^2\div(4\cdot8^2)
\\\\=
(32)^2\div(4\cdot64)
\\\\=
1024\div256
\\\\=
4
.\end{array}