The decay constant of Ra226 is 1.37×10−11s−1. A sample  of Ra226 having an activity of 1.5 millicurie will contain

1 millicurie =3.7×107disintegrations per second1.5 millicurie = 5.55 × 107 disintegrations per secondor 5.55×107N0=K=1.37×10−11∴N0=5.55×1071.37×10−11=4.05×1018