Activity A = λN, where λ is decay constant and N is number of particles present. Therefore, N =
A
λ
= Aτ where τ = 1 / λ is the mean life of the sample. The mass of the sample is M = mN = mAτ where m is mass of an atom. Therefore, the amass of the radioactive sample is M = 10−25×1010×109 = 10−6kg = 1 mg