Let the school be at the origin with coordinates (0, 0).
Rohit walks 30 m North from school:
Position = (0, 30)
Rohit turns left (facing West) and walks 20 m:
Position = (-20, 30)
Rohit turns left again (facing South) and walks 10 m:
Position = (-20, 20)
Rohit turns left again (facing East) and walks 20 m to reach point A:
Position A = (0, 20)
The shortest distance between the school at (0, 0) and point A at (0, 20) is:
d=(0−0)2+(20−0)2
d=0+400
d=400
d=20 m
Therefore, the shortest distance between the school and point A is 20 m.