\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  \frac{dw_i}{dt} = (1 - q_i) \frac{r_i(t-\tau_i)}{w_i} + 
    q_i (-\frac{w_i}{2} r_i(t-\tau_i) ), \qquad
  r_i = \frac{w_i}{\tau_i},
\end{equation*}
\end{document}