Potassium-40 has a half-life of 1.25 billion years - VCE - SSCE Biology - Question 35 - 2015 - Paper 1

Given that potassium-40 has a half-life of 1.25 billion years and the ratio of potassium-40 to argon-40 is approximately 1:1, this indicates that one half-life has elapsed. Therefore, the approximate age of the fossils associated with this layer can be estimated as:

$$ext{Age} ext{ of the fossils} ext{ } ext{≈ half-life} = 1.25 ext{ billion years}$$

Thus, the age of the fossils in the fossil layer will be close to 1.25 billion years.