Answer:
The magnitude of Electric Field is 
Explanation:
Given:
- Radius of the solid sphere=R
- Total charge of the sphere=Q
Let consider a Gaussian surface at a distance of r such that 0<r>R in the shape of sphere such that the electric Field due to this E and it is radially outwards.
The charge inside this Gaussian surface volume we have , 
Now using Gauss Law we have

Yo sup??
Average velocity=total distance covered/total time taken
total distance covered=4 + 8=12 miles
total time taken=6 hours
Therefore
average velocity=12/6
=2 miles/hour
Hope this helps