Acar travels 20 km north. it then turns around and travels 30 km south. what is the car's displacement?

10 km south

Explanation:

The two displacements of the car are vectors, so they have a magnitude and a direction.

The first displacement is 20 km north.

The second displacement is 30 km south.

Since the two displacements are along the same direction (north-south), we can calculate their resultant simply adding them, but taking one of the two with a negative sign (because they are opposite). If we take south as positive direction, and north as negative direction, the resultant displacement becomes

and since it has positive sign, the direction is south.