$A$ radioactive isotope has a half-life of $10 \ days$. If today $125 \ mg$ is left over,what was its original weight $40 \ days$ earlier?

Half-life period of a radioactive element is $10.6 \ yrs$. How much time will it take in its $99 \ \%$ decomposition?

The radioisotope,tritium $({}_1^3H)$ has a half-life of $12.3 \ years$. If the initial amount of tritium is $32 \ mg$,how many milligrams of it would remain after $49.2 \ years$?

The half-life of a radioisotope is $20 \ hours$. After $60 \ hours$,what fraction of the initial amount will remain?

What is the percentage of a radioactive element decayed after $20 \ s$ if its half-life is $4 \ s$?

If $8.0 \ g$ of a radioactive substance has a half-life of $10 \ hrs.$,the half-life of $2.0 \ g$ of the same substance is .......... $hr$.