%% 
%%  Example 07-05-6 
%%  Copyright (C) 2017 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{328.62788pt}
\setlength\parindent{0pt}
\usepackage[T1]{fontenc}
%StartShownPreambleCommands
\usepackage{mathtools}
\def\num#1{\hphantom{#1}}
\def\vsp{\vphantom{\rangle_1}}
%StopShownPreambleCommands
\begin{document}
\begin{equation*}
\frac{300}{5069}%
  \underbrace{\longmapsto\vphantom{\frac{1}{1}}}_{\mathclap{\substack{%
     \Delta a=271\num9\vsp \\[2pt] \Delta b=4579\vsp\\[2pt]  \text{$1$ iteration}}}}
     \frac{29}{490}
     \underbrace{\longmapsto\frac{19}{321}\longmapsto}_{\mathclap{\substack{%
       \Delta a=10\num{9}=\langle271\rangle_{29}\num{20}\\[2pt]
       \Delta b=169=\langle4579\rangle_{490}\\[2pt]
       \text{$2$ iterations}}}} \frac{9}{152}
  \underbrace{\longmapsto \frac{8}{135}\longmapsto\dots\longmapsto}_{\substack{%
       \Delta a=1\num{7}=\langle10\rangle_{9}\num{119}\\[2pt]
       \Delta b=17=\langle169\rangle_{152}\\[2pt]
       \text{$8$ iterations}}} \frac{1}{16}
  \underbrace{\longmapsto\dots\longmapsto\vphantom{\frac{8}{135}}}_{\substack{%
	     \Delta a=0=\langle1\rangle_{1}\num{76} \\[2pt]
	     \Delta b=1=\langle17\rangle_{16} \\[2pt]
             \text{$8$ iterations}}} \frac{1}{1}
\end{equation*}
\end{document}
