The half-life of a radioactive nucleus is 20 hours. What fraction of original activity will remain after 40 hours?

Explanation

`40` hours means `2` half lives.
Thus, `A=(A_(0))/(2^(2))=(A_(0))/(4)`
or `(A)/(A_(0))=(1)/(4)`
So one fourth of the original activity will remain after `40` afters.
Specific activity: The activity per unit mass is called specific activity.