How long will it take for 75.0% of a 0.00970 mg sample of radioactive iron-59 to decay?

Question

The half-life of iron-59 is 45.10 days, so give your answer in days.

Answer 1

In one half life the initial level drops to 50%
In two half lives the initial level drops to 25%.
So you time is equal to 2 half-lives.

Answer 2

90.2 days