Answer:
There are
atoms when the sample of cerium is 20 seconds old.
Explanation:
The decay of isotopes is modelled after the following exponential expression:
(1)
Where:
- Initial number of atoms, dimensionless.
- Current number of atoms, dimensionless.
- Time, measured in seconds.
- Half-time, measured in seconds.
Now we clear the half-time within (1):


If we know that
,
and
, then the half-life of the isotope is:


And the decay equation for the sample is described by:

Lastly, if we get that
, then the activity of the sample of cerium is:


There are
atoms when the sample of cerium is 20 seconds old.