The activity of a radioactive material is $2.56 \times 10^{-3} \,Ci$. If the half life of the material is $5$ days, after how many days the activity will become $2 \times 10^{-5} \,Ci$ ?

A radioactive material decays by simultaneous emissions of two particles with half lives of $1400\, years$ and $700\, years$ respectively. What will be the time after which one third of the material remains? (Take In $3=1.1$ ) (In $years$)

Ther percentage of ${ }^{235} U$ presently on earth is $0.72$ and the rest $(99.28 \%)$ may be taken to be ${ }^{233} U$. Assume that all uranium on earth was produced in a supernova explosion long ago with the initial ratio ${ }^{235} U /^{335} U =2.0$. How long ago did the supernova event occur? (Take the half-lives of ${ }^{235} U$ and ${ }^{238} U$ to be $7.1 \times 10^5$ years and $4.5 \times 10^{9}$ years respectively)

The $S.I.$ unit of radioactivity is

The half life period of a radioactive substance is $5\, min$. The amount of substance decayed in $20\, min$ will be..........$\%$

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