Radioactive half-life — UPSC Physics Optional CSE 1999 Paper II

The specific activity of a sample containing radioactive \text{Co}^{58} and non-radioactive \text{Co}^{59} was observed to be 2.2 \times 10^{12} disintegrations per second per gm. Half-life of \text{Co}^{58} is 71.3 days. Find the percentage of mass of radioactive element in the sample.