Based on mass spectrometer analysis of Uranium-235 and Lead-207, I have determined the age of a zircon to be 89.125 million years old. How many half-lives passed? What are the percentages of Uranium-235 and Lead-207?

Answer 1

half life of U 235: 703.8Million years.

So assuming you are looking at the source of lead to be solely from U235

halflives;89.125/703.8=K

percentU235 left:
percent=100e^(.693*k)

percent o fLead207:
This is kinda hard, as you started with none, so I assume you want the percent of the final amount at time infinity..

Percent=(100-percentU235 left)