Answer
$E=\frac{2500}{\sqrt 3}N/C$
Work Step by Step
Electric flux $\phi$ through any surface is given as
$ \phi= E.A=EAcos\theta$
Here, $\theta=30^{\circ}$, $\phi=25 Nm^{2}/C$ , $A= 2\times 10^{-2}m^{2}$
Thus, $25=E \times (2\times 10^{-2})\times cos 30^{\circ}$
$E =\frac{25}{(2\times 10^{-2})\times cos 30^{\circ}}=\frac{25}{(2\times 10^{-2})\times (\sqrt 3/2)}$
Hence, $E=\frac{2500}{\sqrt 3}N/C$
