Using M. Petrović's \emph{numerical intervals, a system of interval differential equations} of a given system of non-linear differential equations is formed. It is shown that the solutions of the \emph{system of interval equations} approximate the solutions of the given system of non-linear equations. From the \emph{system of interval differential equations} two systems of \emph{boundary equations} is derived, among the solutions of which the solutions of the given system of non-linear equations appear. The graphical solutions of the \emph{system of interval differential equations}, as well as the system of \emph{boundary equations}, can be easily obtained on an analog computer, where the interval containing the solutions of the given system of non-linear differential equations, can be seen. By means of these graphics, the errors appearing when the solutions of the corresponding \emph{system of interval equations} are substituted for the solutions of the given system of non-linear equations, can be estimated.