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.

1'426,000,000 years old

Explanation:

To solve this problem we should keep in mind the definition of half-life: it is the amount of time it takes for a substance to lose half of its radioactivity. For example, if a substance made of 100 radioactive atoms had a half-life of 3 years, then after 3 years there would only be 50 such atoms, then after other 3 years the value would reduce to 25 and so on.

Because in the problem there's only one quarter (1/4) of the original content in the sample, we can deduce that two half-lives have passed, in other words:

713,000,000 years * 2 = 1'426,000,000 years

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