A zircon mineral was analyzed for its Uranium-Lead parent-daughter ratio. Results showed that there was 50% Uranium parent to 50% Lead daughter. How many half lives have elapsed since the mineral crystallized? A. One half of one half-life
B. One half-life
C. Two half-lifes
D. Three half-lifes
E. There is no way to tell given the data provided

One half-life

Explanation:

The ratio of parent-daughter after one half-life is usually 1:1  and after two half-lives 1/4(0.25) of the parent isotope remains and 3/4(0.75) of the daughter isotope remains and becomes the daughter isotope.

Uranium-238 radioactive properties are used to determine the approximate age of mineral rocks. Uranium-238 (half-life = 4.5 109 years) decays to a stable lead-206 isotope.