\begin{tikzpicture}[
  >=latex,
  grow=right,
  edge from parent/.style={draw, thick, blue},
  level 1/.style={sibling distance=4cm, level distance=3cm},
  level 2/.style={sibling distance=2cm, level distance=3cm}
]
\tikzset{
  noeud/.style   = {circle, draw, fill=blue!15},
  prob/.style    = {color=red, fill=white,inner sep=1mm}
}
  \node [noeud] {$\Omega$}
child {
node [noeud] {A}
child {
node [noeud] (f0) {$A_1$}
edge from parent node[prob] {$\dfrac{4}{10}$}
}
child {
node [noeud] {$A_2$}
child {
node [noeud] (f1) {$C_1$}
edge from parent node[prob] {$\dfrac{1}{5}$}
}
child {
node [noeud] (f2) {$C_2$}
edge from parent node[prob] {$\dfrac{4}{5}$}
}
edge from parent node[prob] {$\dfrac{6}{10}$}
}
edge from parent node[prob] {$\dfrac{3}{10}$}
}
child {
node [noeud] {B}
child {
node [noeud] (f3) {$B_1$}
edge from parent node[prob] {$\dfrac{1}{2}$}
}
child {
node [noeud] (f4) {$B_2$}
edge from parent node[prob] {$\dfrac{1}{2}$}
}
edge from parent node[prob] {$\dfrac{7}{10}$}
}
;
\draw[->, thick, orange] (f0.east) -- ++(0.5,0)
  node[right] {\small $\dfrac{3}{25}$};
\draw[->, thick, orange] (f1.east) -- ++(0.5,0)
  node[right] {\small $\dfrac{9}{250}$};
\draw[->, thick, orange] (f2.east) -- ++(0.5,0)
  node[right] {\small $\dfrac{18}{125}$};
\draw[->, thick, orange] (f3.east) -- ++(0.5,0)
  node[right] {\small $\dfrac{7}{20}$};
\draw[->, thick, orange] (f4.east) -- ++(0.5,0)
  node[right] {\small $\dfrac{7}{20}$};

\end{tikzpicture}