Question

The half life of a radioactive substance is $5$ years. After $x$ years a given sample of the radioactive substance gest reduced to $6.25\%$ of its initial value of $x$ is _____

Answer 1

Answer: 20

$T1/2=5$ year
$N=N_0{\left(\frac{1}{2}\right)}^{\text{No of half lives }}$
$\frac{N}{N_0}=\frac{1}{16}={\left(\frac{1}{2}\right)}^4$
Time $=4$ half lives $=20$ years