Determine the activity of the sample of cerium when the sample was 20 seconds old

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.

+15-35=-20

going in opposite directions