With reference to radioactive material, half-life is the time required to 50% depletion of initial amount of material.
Given: Initial amount of radioactive material = 40 g
Half life = 4 days.
Therefore, After 4 days, amount of compound left = 40/2 = 20 g
After 8 days, i.e 2 half-life, amount of compound left = 20/2 = 10 g
Finally after 12 days, i.e. 3 half-life, amount of compound left = 10/2 = 5 g
Thus, 5 grams will remain after 12 days
