As per Newton's III law
Every action force that is applied on an object must be equal and opposite to reaction force applied by that object.
It can be said that when we apply any force on an object then this applied force is known as action force and at the same time that object also apply an opposite direction force on us which is known as reaction force having same magnitude.
So as per above theory
Action Force = Reaction Force (magnitude)
while both forces are always opposite in direction
The electric field strength is inversely related to the square of the distance.so the strength of the electric field is given by
Here, 
is constant depend upon medium and its value is 
and q is charge and r is the distance.
Given 
and we know the charge of proton, 
.
Therefore,
Average Velocity= displacement/time Av=50/0.50 Av=100
