A boy walks 3 km north, then 3 km south. Deepti thinks he has traveled a total of 6 km, so he is…

Why

Deepti is adding up the DISTANCE walked (how much ground was covered in total), but the question is actually asking about DISPLACEMENT (how far the final position is from the starting position) — these two are not the same thing whenever a path doubles back on itself. The boy genuinely did walk 6 km in total, so 'distance travelled' really is 6 km — but south is the exact opposite direction of north, so walking 3 km south after 3 km north cancels the northward movement out entirely, step for step. Test it: picture standing still and taking 3 steps forward, then 3 steps backward, ending exactly where the feet started, even though the legs did real work covering 6 steps' worth of ground. Her friend is right: he ends up back at his exact starting point, 0 km away — walking a distance and ending up displaced from where you started are two different things, and opposite directions can cancel each other out completely.