Answer
$\dfrac{770a}{770-s}$
Work Step by Step
The given expression, $
\dfrac{a}{1-\dfrac{s}{770}}
,$ is equivalent to
\begin{array}{l}\require{cancel}
\dfrac{a}{\dfrac{770}{770}-\dfrac{s}{770}}
\\\\=
\dfrac{a}{\dfrac{770-s}{770}}
\\\\=
\dfrac{\dfrac{a}{1}}{\dfrac{770-s}{770}}
.\end{array}
Using $\dfrac{\frac{a}{b}}{\frac{c}{d}}=\dfrac{a}{b}\div\dfrac{c}{d}=\dfrac{a}{b}\cdot\dfrac{d}{c},$ the expression above is equivalent to
\begin{array}{l}\require{cancel}
\dfrac{a}{1}\cdot\dfrac{770}{770-s}
\\\\=
\dfrac{770a}{770-s}
.\end{array}
