Uranium-235, with a half-life of 713,000,000 years, decays to lead-207. If a rock sample is determined to have one-quarter of the uranium-235 content it had when it formed, the age of the rock sample can be estimated to be approximately ________ years old.
A)178 million
B)713 million
C)28.5 billion
D)1.4 billion

Question

Quizplus · Accepted Answer

The rock has one-quarter of its original uranium-235, meaning it has gone through two half-lives (since 1/2 * 1/2 = 1/4). Each half-life is 713 million years, so 2 half-lives is 1.426 billion years, which is approximately 1.4 billion years.

Uranium-235, with a Half-Life of 713,000,000 Years, Decays to Lead-207