How to solve

A $25C$ point charge is located at the origin. Calculate the electric flux passing through: $($a$)$ that portion of the sphere $r=20cm$ bounded by $=0$ and $,=0$ and $\frac{}{2}$; $($b$)$ the closed surface $=0\mathrm{.}8m,z=+-0\mathrm{.}5$ m ; $($c$)$ the plane $z=4m.[$Ans: $6\mathrm{.}25C,25C,12\mathrm{.}5C]$