Was this perhaps Review Question 15 on page 60 of the Classroom Companion?

In the passage, we are told that the half life of carbon-14 is 5730 years. Let's round that to 5700 years for easier math.

Let's assume today is today! Wow.

If we start with 100% carbon-14, then in 5700 years, we will have 50% carbon-14. In another 5700 years (or a total of 11400 years), we will have 25% carbon-14. In another 5700 years (or a total of 17100 years), we will have 12.5% carbon-14.

In summary:
100% remaining - 0 years
50% remaining - 5700 years
25% remaining - 11400 years
12.5% remaining - 17100 years

We are told that a fossil contains 20% carbon-14.

It must have a lifespan between 11400 and 17100 years.

The answer is EITHER B or C. A and D are eliminated.

Since 20% is approximately halfway between 25% and 12.5%, then the lifespan should be approximately halfway between 11400 and 17100 years. So, C is the best answer!