question with Probability tree

Hi,
I found this file for probability tree:


\documentclass{article}

\usepackage[latin1]{inputenc}
\usepackage{tikz}
\usetikzlibrary{trees}
\usepackage{verbatim}

\begin{comment}
:Title: Probability tree
:Tags: Trees

A simple example of a probability tree diagram. 

\end{comment}


\begin{document}
\pagestyle{empty}


% Set the overall layout of the tree
\tikzstyle{level 1}=[level distance=3.5cm, sibling distance=3.5cm]
\tikzstyle{level 2}=[level distance=3.5cm, sibling distance=2cm]

% Define styles for bags and leafs
\tikzstyle{bag} = [text width=4em, text centered]
\tikzstyle{end} = [circle, minimum width=3pt,fill, inner sep=0pt]

% The sloped option gives rotated edge labels. Personally
% I find sloped labels a bit difficult to read. Remove the sloped options
% to get horizontal labels. 
\begin{tikzpicture}[grow=right, sloped]
\node[bag] {Bag 1 $4W, 3B$}
child {
node[bag] {Bag 2 $4W, 5B$} 
child {
node[end, label=right:
{$P(W_1\cap W_2)=\frac{4}{7}\cdot\frac{4}{9}$}] {}
edge from parent
node[above] {$W$}
node[below]  {$\frac{4}{9}$}
}
child {
node[end, label=right:
{$P(W_1\cap B_2)=\frac{4}{7}\cdot\frac{5}{9}$}] {}
edge from parent
node[above] {$B$}
node[below]  {$\frac{5}{9}$}
}
edge from parent 
node[above] {$W$}
node[below]  {$\frac{4}{7}$}
}
child {
node[bag] {Bag 2 $3W, 6B$} 
child {
node[end, label=right:
{$P(B_1\cap W_2)=\frac{3}{7}\cdot\frac{3}{9}$}] {}
edge from parent
node[above] {$B$}
node[below]  {$\frac{3}{9}$}
}
child {
node[end, label=right:
{$P(B_1\cap B_2)=\frac{3}{7}\cdot\frac{6}{9}$}] {}
edge from parent
node[above] {$W$}
node[below]  {$\frac{6}{9}$}
}
edge from parent 
node[above] {$B$}
node[below]  {$\frac{3}{7}$}
};
\end{tikzpicture}

\end{document}

You can find the link here: https://www.writelatex.com/402223pmfdhy

My
 question is: how can we replace the fractions with a box or circle? I 
am planning to put it as a question to students; I want them to fill out
 the box or circles for the right probabilities.

Thanks,

Wu 
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