Final answer:
The granite is approximately 2800 million years old.
Step-by-step explanation:
The sample of granite contains 25% uranium-235 (parent) and 75% lead-207 (daughter). Uranium-235 has a half-life of 700 million years. To determine the approximate age of the granite, we need to calculate the number of half-lives that have occurred.
Since uranium-235 has a half-life of 700 million years, we divide the age of the sample by the half-life to find the number of half-lives: Age of the granite = (100/25) * 700 million years = 2800 million years.
Therefore, the granite is approximately 2800 million years old.