[DaftJamrock]

'Two solid spheres, 1 and 2, of density 1.0g/cm3 and 10.0 g/cm3, respectively, are in an infinite liquid medium 3 of density 2.0g/cm3 and dielectric constant 2.0. Sphere 1 carries a charge of +e and sphere 2 carries a charge of -e. If the spheres have the same radius R, calculate the value of R for there to be no long-range force between them at any separation D.'

Can somebody help me with this question please?

I think it is based upon Coulomb's law and I know the force equation is q1*q2/4πε0r^2ε. However what I don't understand is how the density comes into this equation.