In 4.0 years, 40.0 kilograms of elemnt a decays to 5.0 kilograms

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.