In basic medium $\mathrm{CrO}_4^{2-}$ oxidises $\mathrm{S}_2 \mathrm{O}_3^{2-}$ to form $\mathrm{SO}_4^{2-}$ and itself changes into $\mathrm{Cr}(\mathrm{OH})_4^{-}$. The volume of 0.154 M $\mathrm{CrO}_4^{2-}$ required to react with 40 mL of $0.25 \mathrm{M} \mathrm{S}_2 \mathrm{O}_3^{2-}$ is $\_\_\_\_$ mL .
(Rounded-off to the nearest integer)