A conducting sphere of radius $10 \mathrm{cm}$ has unknown charge. The electric field at a distance $20 \mathrm{cm}$ from the centre of the sphere is $1.2\times {10}^3 N{C}^{-1}$ and points radially inwards. The net charge on the sphere is

For a point that is outside of the charged sphere, it will act as a point charge with the total charge at centre.

Here, distance of point from the centre of the sphere,

$r=20 cm=0.2 m$

Electric field, $\mathrm{E}=-1.2\times {10}^3 \mathrm{N} {\mathrm{C}}^{-1}$

As $E=\frac{q}{4{\mathrm{\pi \epsilon }}_0{\mathrm{r}}^2}$

Therefore,  $q=\left(4{\mathrm{\pi \epsilon }}_0{\mathrm{r}}^2\right).\mathrm{E}=\frac{{(0.2)}^2\times (-1.2\times {10}^3)}{9\times {10}^9}=-5.3\times {10}^{-9} \mathrm{C}.$