Answer: Average speed is 6 mph and average velocity is 6 mph in north direction.
Explanation:
Since we have given that 
Speed at which Jack is walking = 4 mph
Speed at which Jill is riding a bicycle = 12 mph
So, the average speed would be 

and average velocity would be 
6 mph in the north direction.
Hence, average speed is 6 mph and average velocity is 6 mph in north direction.