Please explain this :

Dear Student ,
From the free body diagram the normal force can be written as:
$$\begin{gathered} N=10\sqrt{3}+\frac{2}{\sqrt{3}}F \\ \Rightarrow \frac{F}{2}=\frac{1}{2\sqrt{3}}\left(10\sqrt{3}+\frac{\sqrt{3}}{2}F\right) \\ \Rightarrow F=20 N \end{gathered}$$
Regards