Two charged spheres separated at a distance d exert a force F on each other. If they are immersed in a liquid of dielectric constant K = 2, then the force (if all conditions are same) is 

Consider the situation shown in the diagram.

Let charges on sphere A and B  are q₁ and q₂, respectively. 
Force between the charges placed in air by Coulomb's law
(spheres can be assumed as poInt charges placed at their centres)

$F_{AB}=k_1\frac{q_1{q}_2}{d^2}$       …(i)

where,    $k_1=\frac{1}{4\pi {\epsilon }_0}$

When the spheres are immersed in a liquid, force between

the charges is  $F{\text{'}}_{AB}^{ }=\frac{q_1{q}_2}{4\pi {\epsilon }_r{d}^2}=\frac{q_1{q}_2}{4\pi {\epsilon }_{0}K}\times \frac{1}{d^2}$

$=\frac{1}{\left(4\pi {\epsilon }_0\right)\times 2}\times \frac{q_1{q}_2}{d^2}=\frac{k_1}{2}\times \frac{q_1{q}_2}{d^2}$             …(ii)

where, K is dielectric constant of the liquid = 2

From Eqs. (i) and (ii), we get  $\frac{F_{AB}}{F_{AB}^{\text{'}}}=\frac{k_1}{k_1/2}=2$

According to question,  $F_{AB}=F$

$\Rightarrow \frac{F}{F{\text{'}}_{AB}^{ }}=2\Rightarrow F{\text{'}}_{AB}^{ }=\frac{F}{2}$