Two identical non-conducting solid spheres of same mass and charge are suspended in air from a common point…

$T\sin \theta =\mathrm{F}; T\cos \theta =mg$

Divide,

$\tan \theta =\frac{F}{mg} ...\left(1\right)$

As force between two charged bodies doesn't depend upon medium. Hence, the force between them remains same because the distance spheres is same.

When it is submerged in liquid then $T^{\text{'}}\sin \theta =\frac{F}{{\epsilon }_r}$

$T^{\mathit{\text{'}}}\cos \theta =mg\left(1-\frac{{\rho }_{\ell }}{{\rho }_{\mathrm{s}}}\right)$

Divide,

$\tan \theta =\frac{F}{\mathit{mg}\left(1-\frac{{\rho }_{\mathcal{l}}}{{\rho }_{\mathrm{S}}}\right)}{\epsilon }_{\mathrm{r}} ...\left(2\right)$

Equating (1) & (2)

$1-\frac{{\rho }_{\ell }}{{\rho }_{\mathrm{s}}}=\frac{1}{{\epsilon }_{\mathrm{r}}} ; 1-\frac{800}{{\rho }_{\mathrm{s}}}=\frac{1}{21}$

${\rho }_s=840 \mathrm{kg} {\mathrm{m}}^{-3}$

$T^{\text{'}}=\frac{T}{{\epsilon }_r}=\frac{T}{21}$