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

The decay of a radioactive element follows first-order kinetics. As a result:

The $T_{1/2}$ of $C^{14}$ isotope is $5770$ years. The time after which $72\%$ of the isotope is left is .......... years.

The half-life for radioactive decay of $^{14}C$ is $5730 \ years$. An archaeological artifact containing wood had only $80\%$ of the $^{14}C$ found in a living tree. The age of the sample is ....... $years$.

What is the half-life of a radioactive substance if $87.5\%$ of any given amount of the substance disintegrates in $40 \, min$?

$1.0 \, g$ of a radioactive isotope was found to reduce to $125 \, mg$ after $24 \, hours$. The half-life of the isotope is $....... \, hours$.