Answer:
9 days before only 8 mg is still emitting radiation.
Explanation:
The exponential model is:

In which a is y(0), that is, the initial quantity.
10 mg is ingested by a patient
This means that
, and thus:

How many days before only 8 mg is still emitting radiation?
This is t for which y(t) = 8. So









9 days before only 8 mg is still emitting radiation.