A fossil was found and determined by radiometric dating to be 11,400 years old. what is the ratio of carbon-14 to carbon-12 in this fossil compared to its environment?

The answer would be 25%

Carbon 14 is radioactive material with 5700 year of half-time which will change it into carbon-12, a more stable substance. If the age of the fossil is 11,400 years old, then the amount of half-time passed would be: 11,400 years / (5,700years / half-time) = 2 half-time.

Since it undergo 2 half-time, the percentage should be: 100% * (1/2) ^2 = 25%