XE % of original parent radiois 50% 25% 12.5% 6.25% 3.125% -50% . How old is this rock sample? Type out your steps in solving this problem. -0% isotope that has become daughter Potassium has a half-life of 1.3 billion years. In a rock sample, there are 5,520 atoms of Argon-40 and 480 atoms of Potassium 40