Final answer:
Sandy, with a negative velocity of -25 km/hr in relation to Mandy's northward velocity, is driving to the south.
Step-by-step explanation:
If Mandy is driving north with a velocity of 50 km/hr and her sister, Sandy, is driving with a velocity of -25 km/hr, then in the context of this problem where north is considered the positive direction, Sandy's velocity being negative indicates that she is driving in the opposite direction of Mandy. Considering typical conventions in physics, if north is positive, south must be negative. Therefore, Sandy is driving to the south.
The answer to the student's question is: Sandy is driving to the south.