A person whose mass is $80\,kg$ is standing over a canoe of $6\,m$ in length and whose mass is $400\,kg$. Both are initially at rest as it is shown in the figure from below. Then the person starts walking from one end to the other of the canoe. Find the distance to the dock when the person has reached the other end of the canoe.

The alternatives given are as follows:

$\begin{array}{ll}

1.&5\,m\\

2.&4\,m\\

3.&1\,m\\

4.&3\,m\\

5.&2\,m\\

\end{array}$

I'm still stuck with this problem. I'm assuming that it has to do with the conservation of the momentum but I don't know how to use it here. Can someone help me here?.