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

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 $C^{14}$ to $C^{12}$ ratio in a wooden article is $13\%$ that of the fresh wood. Calculate the age of the wooden article. Given that the half-life of $C^{14}$ is $5770 \ years$.

$A$ sample is obtained from sea water. $11.9$ $C^{14}$ atoms dissociate from $1 \ g$ of carbon. $15.3$ $C^{14}$ atoms dissociate from a live sample. How many years old is the sample? $(t_{1/2} = 5730 \ \text{years})$

$A$ radioactive isotope has a $t_{1/2}$ of $10 \ days$. If today $125 \ g$ of it is left,what was its weight $40 \ days$ earlier?

If $12 \ g$ of a sample is taken,and $6 \ g$ of the sample decays in $1 \ hr$,the amount of the sample showing decay in the next hour is ........ $g$.