Asample of radioactive material has a half life of 20 years and an activity level of 4 curies. after how many years will the activity level of the sample be at 0.25 curie?

The answer is 80 years.
After 20 years, it will have an activity level of 2.
After 40 years, it will have an activity level of 1.
After 60 years, it will have an activity level of 0.5.
Then, finally, after 80 years the activity level will be 0.25 curie.