The electric field strength decreases as you move farther away from the charge. Since the electric field strength is inversely proportional to the square of the distance from the charge, if the distance is doubled (from 40 cm to 80 cm), the electric field strength will decrease by a factor of four. So, if the electric field strength is initially 8 N/C at 40 cm away, it would be 2 N/C at 80 cm away