Instructions: Amit was driving in New Town, where all roads run either north-south or east-west, forming a grid. Roads are at a distance of 1 km from each other and are parallel.
Amit started at the intersection of streets no. 7 and 8. He drove 3 km north, 3 km west, and then 4 km south. Which further route could bring him back to his starting point? I. 3 km east, then 2 km south II. 1 km north, then 3 km east III. 1 km north, then 2 km west
Show Hint
Track east-west and north-south distance separately; the return route must bring both back to exactly zero.
Instead of coordinates, this can be tracked as a running balance of north-south and east-west distance, which must both return to zero.
North-south balance after the first three moves: $+3$ (north) $-4$ (south) $= -1$, meaning he ends up 1 km south of the start line.
East-west balance after the first three moves: $-3$ (west), meaning he ends up 3 km west of the start line.
To reach zero on both balances, the next moves must add exactly $+1$ to the north-south balance and $+3$ to the east-west balance.
Route I adds $+3$ east and $-2$ south (i.e. $-2$ to north-south), giving a north-south balance of $-1-2=-3$, not zero.
Route II adds $+1$ north and $+3$ east, giving a north-south balance of $-1+1=0$ and an east-west balance of $-3+3=0$: both zero, so this route returns him exactly to start.
Route III adds $+1$ north and $-2$ west, giving a north-south balance of $0$ but an east-west balance of $-3-2=-5$, not zero.
Only route II clears both balances to zero at the same time.