\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,amstext,amsopn,amscd,amsfonts}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
\frac{0.1}{\mbox{OD of culture}} \times \mbox{5 mL}
\end{displaymath}
\end{document}