Final answer:
The sample with 6.25% Uranium-235 and 93.75% Lead is approximately 2.8 billion years old, based on the fact that Uranium-235 has a half-life of 700 million years and the sample has undergone four half-lives.
Step-by-step explanation:
To determine the age of a sample where the percentage of Uranium-235 (U-235) is 6.25% and Lead is 93.75%, we can use the concept of radioactive dating. The half-life of U-235 is 700 million years, which is the time taken for half of the original amount of U-235 to decay into Lead. The current percentages indicate that the original Uranium has gone through several half-lives.
Every half-life reduces U-235 by half. One half-life leaves 50%, two half-lives leave 25%, three half-lives leave 12.5%, and four half-lives leave 6.25%. Since the sample is currently at 6.25%, it has undergone four half-lives.
To calculate the total age of the sample in years, we multiply the number of half-lives by the length of one half-life. Therefore, the age of the rock is 4 half-lives multiplied by 700 million years per half-life, totaling 2.8 billion years.