This problem is requiring us to find the half life of the element. We can solve this problem by using the equation
as shown: A=Aoe^-(ln2/t)T where A is the final amount (5 kilogram), Ao is
the initial amount (40.0 kilogram), t is the half-life (required) and lastly T
time elapsed (4.0 years). Plug in the values in the equation and you should
get 1.33 years as the half life.
