Question

The half-life of radon-222 is 3.8 days. What was the original activity if it has an activity of 10 Bq after 7.6 days?

Answer 1

`40 \ "Bq"`

Explanation:

After `7.6` days, radon-`222` has elapsed `7.6/3.8=2` half-lives. So, its original activity would be `2^2=4` times the amount of activity it has right now.

If it has `10` becquerels of activity right now, that must mean that its original activity was `10 \ "Bq"*4=40 \ "Bq"`.