How many half lives have passed since a particular rock was formed if a sample of the rock contains 31 times as much lead-207 than uranium-235? I know that I need to do a table and calculate I know how to do this question but the thing I need help on is how do I know the number of atoms in order to do a table and calculate. Like I have the ratio of parents:daughter isotope which is 1:31 right? but how do I do this? Just tell me how and I will do it myself. Actually I don't know physics, I'm studying it all by myself and I got this question from my friend's homework and I'm stuck on it, there are 3 questions like it and I need help on how to turn this ratio into actual numbers in order to calculate how many half lives have passed. If I don't need to turn it into numbers then please tell me how do I solve this question? Thank you :) EDIT: okay so thats what I did is this correct? 1 - 31 half lives=5 2 - 30 half lives=4 4 - 28 half lives=3 8 - 24 half lives=2 16 - 16 half lives=1 32 - 0 half lives=0 so 5 half lives have passed Correct??