Answer: 60m
Step-by-step explanation:
Displacement is defined as the distance covered by a body or object in a specified direction. 
If the dog runs 80m northwards i.e +80m since it runs along the positive y direction. 
Also the dog returns to its master 20m in the south direction i.e -20m in the negative y direction. 
Its total displacement will be +80-20 = 60m