\documentclass{article}
\usepackage{tikz}
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{flushleft}
\setlength\PreviewBorder{35pt}%
 

% colors for the ruled rectangle
\definecolor{color1}{RGB}{249,255,195}
\definecolor{color2}{RGB}{251,245,160}
% colors for the fulcrum
\definecolor{color3}{RGB}{239,178,107}
\definecolor{color4}{RGB}{214,96,0}
% colors for the weights
\definecolor{color5}{RGB}{191,203,224}
\definecolor{color6}{RGB}{105,103,161}

% A pic path for the weights
\tikzset{
Peso/.pic={
  \coordinate (wll) at (-0.4cm,0);
  \coordinate (wlr) at (0.4cm,0);
  \coordinate (wul) at (-0.3cm,0.4cm);
  \coordinate (wur) at (0.3cm,0.4cm);
  \draw[fill=color5] 
    (wll) -- (wlr) -- (wur) -- (wul) -- cycle;
  \draw[fill=color6]
    (wur) --
    ++(-\Angle:6pt) --
    ++(-75:0.4cm) --
    (wlr) --
    cycle;  
  }
}

% counters to store the number of weights and the sum of
% the given integers
\newcounter{Number}
\newcounter{Suma}

% to perform a scaling along the x-axis (1cm=no scaling)
\newlength\XFactor
\setlength\XFactor{1cm}

% the height of the rectangle
\def\RectHt{1cm}
% the angle for the 3D effect
\def\Angle{20}
% lengths controlling the help lines separation
\def\Corr{1pt}
\def\Gap{6pt}

% the main environment
\newenvironment{Balance}[1]
{%
  \medskip\par\noindent
  \setcounter{Number}{0}%
  \setcounter{Suma}{0}%
  \begin{tikzpicture}[x=\XFactor]
    \foreach \Valor in {#1}
    {
      \pic at (\Valor,\RectHt) {Peso};
      \stepcounter{Number}%
      \addtocounter{Suma}{\Valor}
      \xdef\Steps{\Valor}
    }
    \def\RectWd{\numexpr\Steps+1\relax*\XFactor}%
    \pgfmathsetmacro{\FulcrumCoord}{\theSuma/\theNumber}

% The ruled rectangle
    \coordinate (ll) at (0,0);%\node at ([xshift=-10pt]ll) {\FulcrumCoord};
    \coordinate (ur) at (\RectWd,\RectHt);
    \draw[fill=color1]
      (ll) rectangle (ur);
    \draw[fill=color2] 
      (ur) -- 
      ++(-\Angle:6pt) -- 
      ++(0,-\RectHt) -- 
      ++(-\RectWd,0) -- 
      (ll) --
      ++(\RectWd,0) --
      cycle;   
    \foreach \Valor in {1,...,\Steps}
      \draw (\Valor,1) -- ++(0pt,-5pt) node[below] {\Valor};  
    \draw (\RectWd,0) -- ++(-\Angle:6pt);

% The fulcrum
    \draw[fill=color3] 
      (\FulcrumCoord,0) --
      ++(240:1) --
      ++(1,0) coordinate (aux) --
      cycle;
    \draw[fill=color4]  
      (\FulcrumCoord,0) --
      ++(-\Angle:6pt) --
      ++(-60:1) --
      ++(-1,0) --
      ++(180-\Angle:6pt) --
      ++(1,0) --
      cycle;
    \draw (aux) -- ++(-\Angle:6pt);
    \coordinate (cotaFulc) 
      at (\FulcrumCoord,\RectHt);
    \draw[color4] 
      (\FulcrumCoord,0) --
      ++(0,2pt) 
        node[above=-2pt] {\pgfmathprintnumber[precision=1]{\FulcrumCoord}};
}
{\end{tikzpicture}\medskip\par\noindent}

% the command to draw help lines
\newcommand\Cotas[2][1]{
\draw 
  (\FulcrumCoord,\RectHt+#1\Corr+\number\numexpr#1-1\relax\Gap) -- 
  ++(0,10pt);
  \foreach \Valor in {#2}
    {
      \draw 
        (\Valor,\RectHt+#1\Corr+\number\numexpr#1-1\relax\Gap) -- 
          coordinate (cota\Valor) 
        ++(0,10pt);
      \draw[<->,>=latex,help lines] 
        (cotaFulc|-cota\Valor) -- 
          node[fill=white] {\footnotesize$\pgfmathparse{\Valor-\FulcrumCoord}%
          \pgfmathprintnumber[precision=1]{\pgfmathresult}$} 
        (cota\Valor);
    }
}
\begin{document}
\begin{flushleft}
\begin{Balance}{3,4,8}
  \Cotas{4,8}
  \Cotas[2]{3}
\end{Balance}

\begin{Balance}{2,4,6,8,10}
  \Cotas{4,8}
  \Cotas[2]{2,10}
\end{Balance}

\begin{Balance}{1,4,5,8,10,12}
  \Cotas{5,8}
  \Cotas[2]{4,10}
  \Cotas[3]{1,12}
\end{Balance}
\end{flushleft}
\end{document}