For a soap bubble floating in air, Gravitational force $=$ Buoyant force

$\Rightarrow g$ (mass of helium $+$ mass of soap film) $=$ Weight of air displaced by bubble $\ldots$ $(i)$

Let $r=$ inner radius of soap bubble and $t=$ thickness of film.

Then, from Eq. $(i)$, we have

$\Rightarrow \quad \frac{4}{3} \pi r^3 \times \rho_{ He }+4 \pi r^2 \times t \times \rho_{\text {soap }}$ $=\frac{4}{3} \pi r^3 \times \rho_{\text {air }}$

Substituting values in above equation, we get

$\Rightarrow \frac{4}{3} \times \pi \times\left(10^{-2}\right)^3 \times 0.18+4 \pi \times\left(10^{-2}\right)^2$ $\times t \times 1000$

$=\frac{4}{3} \times \pi \times\left(10^{-2}\right)^3 \times 1.23$

Rearranging, we get

$\Rightarrow 4 \pi\left(10^{-2}\right) \cdot t \cdot 1000=\frac{4}{3} \pi\left(10^{-6}\right)(108)$

$\Rightarrow \left(10^5\right) t=0.35$

or $t=3.5 \times 10^{-6} \,m$

$=3.50 \,\mu m$