This is a problem involving half-lives. We can solve this problem by using the equation as shown: A=Aoe^-(ln2/t)T where A is the final amount (1.2milligram), Ao is the initial amount (19.2mg), t is the half-life (4.5 days) and lastly T time elapsed (required). Plug in the values in the equation and you should get 18 days. Eighteen days have passed.