Two positively charged objects are located so they are 0.20 m apart. If the objects are separated until their centres are 0.40 m apart, by what factor will the electrostatic force between them change?

The electrostatic force is inversely proportional to the square of the distance that separates the center of the charges. Then if the distance is multiplied by 2, the force will be divided by a factor of 2^2 = 4 or multiplied by 1/4.