In the given figure if the magnitude of force applied by +Q charge on +q charge is F. Then the net electric force on +Q charge is n F. What is the value of n?

Let force between +q and +Q be F, then the effect of all charges on +Q can be seen in the given figure.

By symmetry we can see that the components of 5F perpendicular to 4F will cancel out, only remaining will be along the direction of 4F. which will be equal to

$\Rightarrow F_{\text{net }}=4F+5F\cos \left({60}^{\circ }\right)+5F\cos \left({60}^{\circ }\right)$

$\Rightarrow {\mathrm{F}}_{\mathrm{net}}=4\mathrm{F}+\frac{5\mathrm{F}}{2}+\frac{5\mathrm{F}}{2}=9\mathrm{F}$

hence, n = 9.