No of moles:-
[tex]\\ \sf\longmapsto \dfrac{2\times 10^{21}}{6.022\times 10^{22}}[/tex]
[tex]\\ \sf\longmapsto 0.3\times 10^{-1}mol[/tex]
[tex]\\ \sf\longmapsto 0.03mol[/tex]
Now
0.03mol of element weigh 0.4g
[tex]\\ \sf\longmapsto \dfrac{0.4}{0.03}[/tex]
[tex]\\ \sf\longmapsto 13.3g[/tex]
Now
[tex]\\ \sf\longmapsto 13.3(0.5)=6.65g[/tex]