Question

A car moves 100m due east and then 25m due west.
(a) what is the distance covered?
(b) what is the displacement?

Answer 1

distance covered 125m
total displacement 75m

Answer 2

Short answer:
a) 100+25 =125m 
b) 100-25=75m

Further explanation: 
Distance is a scalar quantity, only look for magnitude. While displacement is a vector quantity, it look for magnitude and direction. 
When direction is in condition, the car move from a starting point, moving to the east → 100m and move to west 25m which means move to the left 25m.
The car is actually moving backward and so need to deduct 100 by 25.