The electric charges are distributed in a small volume. The flux of the electric field through a spherical surface of radius 10 cm surrounding the total charge is 20 V-m. The flux ( in V-m ) over a concentric sphere of radius 20 cm will be

According to Gauss's law, total flux coming out of a closed surface enclosing charge q is given by  $\varphi =\int \mathbf{E}·d\mathbf{S}=\frac{q}{{\epsilon }_0}$

From this expression, it is clear that total flux linked with a closed surface only depends on the enclosed charge and independent of the shape and size of the surface

$\varphi =\int \mathbf{E}·d\mathbf{S}=\frac{q}{{\epsilon }_0}=20\mathrm{Vm}$  [given)

This $\frac{q}{{\epsilon }_0}$ is constant as long as the enclosed charge is constant. 
The flux over a concentric sphere of radius 20 cm = 20 V-m.