\documentclass{article}
\usepackage{xepersian}
\settextfont[Language=Default]{Persian Modern}
\begin{document}
\begin{enumerate}
\item سلام
\begin{LTRitems}
\addfontfeature{Script=Latin} % or \resetlatinfont if you prefer latin digits. note that this should be inside a group, or environment; i.e. it is done here inside LTRitems environment
\item $a+b+c+d=j$
\item $e+g+f+d=t$  
\end{LTRitems}
\end{enumerate}
\end{document}