An igneous rock contains one half of its original amount of potassium 40 the age of the igneous rock is closest to what

So we want to know what is the age of igneous rock if it contains one half of its original amount of potassium 40. The half-life of potassium 40 is 1.251*10^9 years. Since in the igneous rock there is one half of potassium left it means only one half-life has passed. And its age is 1.251*10^9 years or 1.251 billion years.