Compute 

(a) The electric field E in air at a distance of 30 cm from a point charge q•₁ = 5.0 × 10⁻⁹ C, 

(b) The force on a charge q•₂ = 4.0 × 10⁻¹⁰ C placed 30 cm from q•₁, and 

(c) The force on a charge q•₃ = −4.0 × 10⁻¹⁰ C placed 30 cm from q•₁ (in the absence of q•₂).