Let us use the conventional directions, as shown in figure, to solve this problem.
According to question, let us draw the diagram.
There are two possible diagrams.
Clearly,
‌‌CE=AB=30m⇒CD=30√2m Using Pythagoras theorem,
(CE)2+(DE)2=(CD)2 ⇒(DE)2=(30√2)2−(30)2=1800−900=900m
⇒‌‌DE=30m Now, it is clear that Case I is not possible.
Using Case II,
Clearly, displacement of the person is
End point - Start point
=DA  (along West)
and
‌‌DA=DE−AE =30m−20m=10m (along West)