Mass of man = M
mass of plank = M/3
let man is standing at the starting of plank(end A), at x=0
end of plank(end B) is at x=L
CM of man is at x=0
CM of plank is at x=L/2
position of CM of system =
Let after walking to end A from end B, he travels a distance z.
CM of man is at x=z
CM of plank will shift (L-z) towards left,
CM of plank is at = [L/2 - (L-z)] = (z-L/2)
Since there is no external force, CM of system won't change its position.
So answer is L/4