\documentclass{beamer}
\usepackage{accents}
\usepackage{pgffor}
\begin{document}
\begin{frame}[fragile]{}
\begin{itemize}[<+(1)->]
\item one
\item two
\item three
\end{itemize}
\pause
First line:
\[\edef\myoffset{\number\value{beamerpauses}}
\foreach \x [count=\y from \myoffset] in {1,2,3,4,5,6,7,8,9}{
\alt<\y>{\underaccent{\bar}{\x}}{\x}
}
\]
\vskip 0.5cm
\hspace*{1cm}\edef\myoffsetB{\number\value{beamerpauses}}
$\begin{aligned}
S &= \foreach [count=\y from \myoffsetB] \x in
{{\{1\}},{\{1,2\}},{\{1,2,3\}},{\{1,4,3\}},{\{5,4,3\}},{\{5,6,3\}},{\{5,6,3\}},{\{5,6,3\}},{\{5,6,9\}}}{
\only<\y>{\x}}\\
\frac{s}{t} &=
\foreach [count=\y from \myoffsetB] \x in
{1,1,1,0.75,0.6,0.5,0.429,0.375,0.333}{\only<\y>{\x}}
\end{aligned}$
\end{frame}
\end{document}