Question

You have a meteorite sample and you decide to use the uranium-235/lead-207 system to date it. After analysis, you find that it has 22,500 atoms of 235U remaining, and 1,477,500 atoms of 207Pb that the 235U decayed to. What percentage of the original amount of 235U is still present?

Answer 1

Originally there must been

1,4775E6 + 2.25E4 = 147.75E4 + 2.25E4 = 150E4 present at start

% = 2.25 / 150 = 1.5 %      of 235 U left