\(\sigma \frac{ V }{2} g+2 \pi R T=\rho Vg\)
\(2 \pi RT =\frac{(2 \rho-\sigma)}{2} \frac{4}{3} \pi R ^{3} g ; \quad\left[ V =\frac{4}{3} \pi R ^{3}\right]\)
\(R ^{3}=\frac{3 T }{(2 \rho-\sigma) g} \Rightarrow R =\sqrt{\frac{3 \times 7.5 \times 10^{-2} N - m ^{-1}}{(2 \rho-\sigma) \times 10}}\)
\(R=\frac{3}{20 \sqrt{(2 \rho-\sigma)}} m =\frac{15}{\sqrt{2 \rho-\sigma}}\,\,cm\)