Apply the relation between half life and radiactive decay constant,
[tex]\lambda=\ln(2)/T_{1/2}[/tex]
Then, use the relation of radioactive desintegration,
[tex]N=N_0e^{-\lambda t}[/tex]
In this case, using the mass as N0, you will obtain,
[tex]N=6.25\ \ g[/tex]
