% Dans le préambule
% -----------------
\usepackage{lcg,calc}
\usepackage{amsmath}
% -----------------

\newcommand{\randfrac}{
\reinitrand[first=1,last=10,counter=num] \rand
\reinitrand[first=2,last=20,counter=deno]\rand
\reinitrand[first=2,last=5, counter=coef]\rand
\setcounter{num}{\value{coef}*\value{num}}
\setcounter{deno}{\value{coef}*\value{deno}}
$\dfrac{\thenum}{\thedeno}$
}
Simplifier la fraction : \randfrac
\medskip

Simplifier la fraction : \randfrac
\medskip

Simplifier la fraction : \randfrac
\medskip

Simplifier la fraction : \randfrac