In the reported figure, two bodies AandB of masses 200g and 800g...

$\omega = \sqrt{\dfrac{{k}_{eq}}{\mu}}$

$\mu =$ reduced mass

Springs are in series connection,

${k}_{eq} = \dfrac{{k}_{1}{k}_{2}}{{k}_{1} + {k}_{2}}$

${k}_{eq} = \dfrac{k \times 4k}{5k} = \dfrac{4k}{5}$

${k}_{eq} = \dfrac{4 \times 20}{5}N{m}^{- 1} = 16N{m}^{- 1}$

$\mu = \dfrac{{m}_{1}{m}_{2}}{{m}_{1} + {m}_{2}} = \dfrac{0 . 2 \times 0 . 8}{0 . 2 + 0 . 8} = 0 . 16kg$

$\omega = \sqrt{\dfrac{16}{0 . 16}} = \sqrt{100} = 10$