The half-life of cobalt-60 is 5 years. How old is a sample of cobalt-60 if only one-eighth of the original sample is still cobalt-60?

5 years

10 years

15 years

20 years

Answer: C

15 years old

Explanation:

Given that the half life = 5 years

Let No = initial mass

N = mass after decay

Initially at t = 0

N = No

After t = 5

N = No/2

After t = 10

N = No/4

After t = 15

N = No/8

But the question ask How old is a sample of cobalt-60 if only one-eighth of the original sample is still cobalt-60?

Therefore, original sample is 15 years old.