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

The radioactivity due to the $^{14}C$ isotope (half-life $6000 \ years$) of a sample of wood from an ancient tomb was found to be nearly half that of fresh wood. The tomb is therefore about ........... years old.

$A$ first-order nuclear reaction is half-completed in $45 \ minutes$. How long does it take for $99.9\%$ of the reaction to be completed in $hours$?

$87.5\%$ decomposition of a radioactive substance completes in $3$ hours. What is the half-life of that substance?

$A$ radioactive element with half-life $6.5 \ hr$ has $48 \times 10^{19}$ atoms. Number of atoms left after $26 \ hr$ is:

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