Force between the balloons can be calculated using Coulomb's law. Coulombs law states that the force of attraction or force of repulsion between 2 point charges is proportional to the product of the values of the charges and inversely proportional to the square of the distance between them.
F = kq₁q₂/r²
where F - force, if its 2 like charges then its force of repulsion, if its unlike charges then force of attraction
q₁ - point charge 1
q₂ - point charge 2
r - distance between 2 charges
substituting these values
F = (4.0 × 10⁻⁶ C x 8.2 × 10⁻⁶ C x 9.0 × 10⁹ Nm²/C²) / (2 m)²
= 295.2 x 10⁻³ / 4
= 7.38 x 10⁻² N