\documentclass{article}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{amsfonts}
\usepackage{graphicx}


\begin{document}

A powerhouse is on one edge of a straight river and a factory is on the other edge, $100$ meters 
downstream.  The river is $50$ meters wide.  It costs $10$ per meter to run electrical cable across 
the river and $7$ per meter on land.  How should the cable be installed to minimize the cost?
\end{document}