The half life of Pb-210 is 22 years. A decayed animal shows 25% of the original Pb-210 remains; how long has the animal been deceased to the nearest tenth of a year?

This is an easy half-life calculation. (Most of these are NOT easy).

If 25% remains of an original amount of 100%, then the time elapsed is precisely 44 years. (2 half-lives)

In 22 years (one half-life), it would be 50%

In 44 years (two half-lives), it would be 25%

In 66 years (three half-lives), it would be 12.5%

In 88 years (four half-lives), it would be 6.25%