The point charges 3μC and 4μC are placed at a separation of 7 m.  The medium between them is of two types as shown in figure, the electric force acting between them is

For finding air equivalent separationWe have 14πϵ0q1q2r′2=14π∈0∈rq1q2r2      F′=14π∈0q2q2r′2⇒r′=ϵrr       =1.73×10−4N∴requivalent =(9)3+(16)4=25mTherefore , F' =14πε0 q1q2requivalent2=9×109×3×4×10-12252 N =1.732×10-4 N