When we calcuate a numerical age date we examine the ratio of parent radioisotope to the decay products. If we begin with 128 atoms of isotope, step through the first several half-lives and track the ratio of parent to decay atoms. First the system closes After first half-life the P:D ratio is 50:50 and this simplifies to 1:1 After second half-life the P:D ratio is? and this simplifies to 1:3 After third half-life the P:D ratio is ? and this simplifies to 1:7
