marbles.tex

\documentclass[11pt]{article}
\usepackage[margin=1in]{geometry}

\usepackage{amsmath}

\frenchspacing

\begin{document}

\begin{tabular}{l c l}
The number of yellow marbles & : & $y$ \\
The number of expensive yellow marbles & : & $y_e$ \\
\emph{Every yellow marble is expensive} & : & $y = y_e$ \\ \\

The number of orange marbles & : & $o$ \\
The number of expensive orange marbles & : & $o_e$ \\
\emph{One-half the orange marbles are expensive} & : & $\frac{1}{2}o = o_e$ \\ \\

The total number of expensive marbles & : & $m_e$ \\
\emph{One-half of all expensive marbles are yellow} & : & $\frac{1}{2}m_e = y_e$ \\ \\

\emph{There are 40 yellow marbles} & : & $y = 40$ \\ \\

The number of expensive marbles that are neither yellow nor orange & : & $m_e - y_e - o_e$ \\
\emph{There are 25 expensive marbles that are neither yellow nor orange} & : & $m_e - y_e - o_e = 25$ \\ \\

\emph{How many orange marbles are there?} & : & $o = \; ?$

\end{tabular}

\hfil

\begin{equation}
y = y_e \implies y_e = 40
\end{equation}
\begin{equation}
\frac{1}{2}m_e = y_e \implies m_e = 2y_e \implies m_e = 80
\end{equation}
\begin{equation}
m_e - y_e - o_e = 25 \implies 40 - o_e = 25 \implies o_e = 15
\end{equation}
\begin{equation}
\frac{1}{2}o = o_e \implies o = 2o_e \implies o = 30
\end{equation}

\hfil

There are 30 orange marbles.

\end{document}