documentclass[varwidth]{standalone}
usepackage{expl3}
begin{document}
  ExplSyntaxOn
    % following is 60.0ptmm (which is 300/5,
    %  not to mention silly units; ridiculous)
    dim_eval:n{300pt/5mm}\

    % following is 14.22636pt (I guess/hope it's correct
    %  conversion of 5mm to pt units)
    dim_eval:n{5mm}\

    % following is 21.42857pt (which is 300/14,
    %  not 300/14.22636)
    %   followed by .22636pt (decimal of 14.22636pt;
    %    which is also ridiculous)
    dim_eval:n{300pt/14.22636pt}
  ExplSyntaxOff
end{document}

dim_eval:n{300pt/5mm}

dim_eval:n{300pt/5}mm

dim_eval:n{300pt/14.22636pt}

dim_eval:n{300pt/14}.22636pt

documentclass[varwidth]{standalone}
usepackage{expl3}
begin{document}
  ExplSyntaxOn

    fp_eval:n{(300pt)/(5mm)}

   ExplSyntaxOff
end{document}