Refer to the radioisotope colbalt-60, which has a half life of 5.3 years. An initial sample of colbalt-60 has an activity of 12,400cpm. How much time has passed if the activity of the sample has dropped to 775cpm

21.2 years

Explanation:

If half life is 5.3 years, that means activity will be 50% every 5.3 years.

12,400 x 50% = 6,200

6,200 x 50% = 3,100

3,100 x 50% = 1,550

1,550 x 50% = 775

4 half lives, so 4 half lives x 5.3 years/half life = 21.2 years