The half life of 131I is 8 days. Given a sample of 131I at time t = 0, we can assert that  

[d] Number of nuclei decreases exponentially$N=N0e-\lambda t\text{and}Rateofdecay\left(-dNdt\right)=\lambda N$Therefore, decay process losts up to t = ¥.\displaystyle{T}{h}{e}{r}{e}{f}{\quad\text{or}\quad}{e},{a}{g}{i}{v}{e}{n}\nu{c}\le{u}{s}{m}{a}{y}{d}{e}{c}{a}{y}{a}{t}{a}{n}{y}{t}{i}{m}{e}{a}{f}{t}{e}{r}{t}={0}.