Physics - Modern Physics Question with Solution | TestHub

Here, $t=25$ years, $T_{1/2}=5$ years Number of half lives, $n=\frac{t}{T_{1/2}}=\frac{25}{5}=5$ Percentage of radio active material remained after $$\begin{array}{rl}25\text{ years is }& ={\left(\frac{1}{2}\right)}^n\times 100\\ & ={\left(\frac{1}{2}\right)}^5\times 100=3.125\%\end{array}$$