Question

At a given instant , $60\%$ of the radioactive nuclei in a sample are left undecayed. After $20$s, $85\%$ nuclei have disintegrated, mean life of nuclei

• A. $10$s
• B. $6.93$s
• C. $14.43$s
• D. $28.86$s

Answer 1

The correct option is D $28.86$s

From the equation it is clear that half life if radioactive nuclei is to see,

Mean life $=\frac{1}{\lambda }$

$=\frac{1}{0.693}$

$=\frac{t_{1/2}}{0.693}$

$=\frac{20}{0.693}$

$=28.86s$

$\therefore$ Mean life $=28.86s$.