Two blocks of masses 3kg and 5kg are connected by a metal wire...

Tension in the string $= \dfrac{2{m}_{1}{m}_{2}}{\left({m}_{1} + {m}_{2}\right)}g$

$= \dfrac{2 \times 3 \times 5 \times 10}{8} = \dfrac{75}{2}$

Stress $= \dfrac{T}{A}$

$\dfrac{24}{\pi} \times {10}^{2} = \dfrac{75}{2\left(\pi{r}^{2}\right)}$

${r}^{2} = \dfrac{75}{48} \times {10}^{- 2}$

${r}^{2} = 0 . 015625$

$r = 0 . 125m = 12 . 5cm$