-- The overall distance he travels is (100m + 30m + 70m) = 200 meters.
-- His displacement when he arrives at his front door is
D = (100m East) + (30m West) + (70m East)
D = (100m + 70m)East + (30m)West
D = (170m East) + (30m West)
D = 140 meters East
It's interesting to notice that his displacement is 60 meters shorter than the distance he walked.
That's because there's a stretch of 30 meters somewhere in the middle that he actually covered three times.
Two of those times added to the distance his shoes covered (2x30m=60m), but they cancelled out of the displacement.
His front door is 140 meters East of school. He walked 60m farther than that, going back and forth over the 30m piece.
