A2,000 g quantity of c-14 is left to undergo radioactive decay. the half-life of c-14 is approximately 5,700 years. after going trough x half-lives, the sample becomes 125 grams. what is x? a) 22,800 b) 28,500 c) 4 d) 5

The half-life is the time the sample takes to reduce to half of its original value. If we call  the initial mass of the sample, this means that after 1 half-life the mass will be , after 2 half-lives the mass will be , and so on..
Therefore, after x half-lives the mass of the sample will be
(1)
In our problem, the initial mass is  while the mass after x half-lives is , so by using equation (1) we can find the value of x:

From which

And the correct answer is C).

It is d, all the other ones say incorrect answer .