\DeclareMathOperator{\const}{\text{상수}}

\usepackage{amsmath}%
\DeclareMathOperator*{\plus}{+}

\documentclass{article}
\usepackage{amsmath}
\usepackage{hfont}
\newcommand\dcbracket[1]{『#1』}%
\DeclareMathOperator{\dalmeum}{\text{\lower.2ex\hbox{\large ∽}
                                     }}%
\DeclareMathOperator*{\haneul}{\text{\LARGE 天}}%
\DeclareMathOperator*{\ddang}{\text{\LARGE 地}}%

\begin{document}
\begin{equation*}
 \triangle ABC \dalmeum \triangle DEF
\end{equation*}
\centering{\Large \dcbracket{天地}}
\begin{equation*}
 \haneul_{\text{천}}^{\text{하늘}} \quad
    \ddang_{\text{지}}^{\text{땅}}
\end{equation*}

\end{document}

I know how to write like this.
 
 5    16    2
 -- X -- = --
 8    15    3
 
 But I want to describe the process of dividing out common factors from the 
 numerators and denominators.
 For example, there should be '1' above 5 and '3' below 15, and so on.
 How do I cross out the letters and write a little numbers above or below the 
 numerators and denominators.. 

\documentclass[12pt]{article}
\usepackage{cancel}
\usepackage{amsmath}

\begin{document}
\[
\frac{\,\mathop{\cancel{5}}\limits^{1}\,}{\,\mathop{\cancel{8}}\limits_{1}\,}
 \times
 \frac{\,\mathop{\cancel{16}}\limits^{2}\,}{\,\mathop{\cancel{15}}\limits_{3}\,}
 =\frac{2}{3}
\]
$$ {\mathop{\cancel{5}}^{1}}\quad ... \quad {\mathop{\cancel{15}}_{3}} $$
\end{document}