From the principle of exponential radioactive decay,

we know that

Here, the mass of the isotope is = 15 g

No. of atoms in 15gm of atom is = atoms

And, λ = (0.693/T_1/2) where, Half life = 28 × 365 × 86400 = 8.83 × 10⁸ secs

Hence,

The disintegration rate of 15 mg of the isotope will be