A certain radioisotope X (T₅₀ =10 days) decays to give $\left(\begin{array}{l}\mathrm{A}-4\\ \mathrm{Z}-2\end{array}\mathrm{Y}\right)$ If one g atom of $\left({ }_{\mathrm{Z}}^{\mathrm{A}}\mathrm{X}\right)$ is kept in a sealed vessel, how much helium will accumulate in 20 days at STP?