Consider the integral form of the Gaussian law in electrostatics: $\oint \vec{E} \cdot d\vec{s} = \frac{Q}{\epsilon_0}$. Which of the following statements are correct?

When does the electric flux associated with a closed surface become positive,zero,or negative?

Consider a region in free space bounded by the surfaces of an imaginary cube having sides of length $a$ as shown in the figure. $A$ charge $+Q$ is placed at the centre $O$ of the cube. $P$ is a point outside the cube such that the line $OP$ perpendicularly intersects the surface $ABCD$ at $R$, and $OR = RP = a/2$. $A$ charge $+Q$ is also placed at point $P$. What is the total electric flux through the five faces of the cube other than $ABCD$?

What kind of Gaussian surface is used to calculate the electric field due to the charge distribution?

Gauss's law states that

$A$ rectangular surface of sides $10 \,cm$ and $15 \,cm$ is placed inside a uniform electric field of $25 \,V/m$,such that the surface makes an angle of $30^{\circ}$ with the direction of the electric field. Find the flux of the electric field through the rectangular surface in $Nm^2/C$.