Complète
  \par\vspace{3mm}\par
  \[\Eqalign{
    \frac{\rnode{A}{21}}{\rnode{B}{12}}\quad&=
    \quad\frac{\ldots\times\ldots}{\ldots\times\ldots}\quad=
         \quad\frac{\rnode{C}{\strut\ldots}}{\rnode{D}{4}}\kern1cm
    &\frac{\rnode{E}{8}}{\rnode{F}{20}}\quad&=
    \quad\frac{\ldots\times\ldots}{\ldots\times\ldots}\quad= 
        \quad\frac{\rnode{G}{\strut\ldots}}{\rnode{H}{5}}\kern1cm&
    \frac{\rnode{I}{24}}{\rnode{J}{\strut\ldots}}\quad&
    =\quad\frac{\ldots\times\ldots}{\ldots\times\ldots}\quad=
        \quad\frac{\rnode{K}{2}}{\rnode{L}{3}}\cr
    \cr
    \cr
    \frac{21}{35}\quad&=\quad\frac{\ldots\times\ldots}{\ldots\times\ldots}\quad=
    \quad\frac{3}{\ldots}&\frac{\ldots}{24}\quad&\quad
    \frac{\ldots\times\ldots}{\ldots\times\ldots}\quad=
    \quad\frac23&\frac{48}{42}&=\quad
    \frac{\ldots\times\ldots}{\ldots\times\ldots}\quad= \quad\frac{\ldots}{7}\cr
  }\]
  \ncarc[arcangle=60,nodesep=1mm]{->}{A}{C}
  \ncput*{\small$\div\ldots$}
  \ncarc[arcangle=-60,nodesep=1mm]{->}{B}{D}
  \ncput*{\small$\div\ldots$}
  \ncarc[arcangle=60,nodesep=1mm]{->}{E}{G}
  \ncput*{\small$\div\ldots$}
  \ncarc[arcangle=-60,nodesep=1mm]{->}{F}{H}
  \ncput*{\small$\div\ldots$}
  \ncarc[arcangle=60,nodesep=1mm]{->}{I}{K}
  \ncput*{\small$\div\ldots$}
  \ncarc[arcangle=-60,nodesep=1mm]{->}{J}{L}
  \ncput*{\small$\div\ldots$}