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 (1gram *assumed), Ao is the initial amount (16g *assumed), t is the half-life *required) and lastly T time elapsed (26.76h). Plug in the values in the equation and you should get 6.69 hours.
