1total alpha particles released=2.24*10-3*60*24*420=x suppose
therefore total atoms released=x
now after 420 days
pv=nRt
where p=pressure
v=volume
n=no. of moles
R=universal gas consntant
t=temperature
from here we calculate n=pvrt
now, in n moles we get x atoms
therefore in 1 mole we get xn atoms :-)