We can solve this problem by using the equation as shown: A=Aoe^-(ln2/t)T where A is the final amount (0.06 gram), Ao is the initial amount (1g *assumed), t is the half-life (10 years ) and lastly T time elapsed (required). Plug in the values in the equation and you should get 40.5 years and this is approximately 4 half-lives.
