A flat, square surface with sides of length L is described by the equations x=L,  0≤y≤L,  0≤z≤L The electric flux through the square due to a positive point charge q located at the origin (x=0,  y=0,  z=0) is

Imagine a charge q at the center of a cube of edge length 2L (figure). (figure). Then ϕ=q/ε0. Here, the square is 1/24 of the surface area of the imaginary cube, so it intercepts 1/24 of the flux. That is, Φ=q24ε0