Question

Radioactive​ uranium-235 has a​ half-life of about 700 million years. Suppose you find a rock and chemical analysis tells you that only one sixteenth
of the​ rock's original​ uranium-235 remains. How old is the​ rock?

Answer 1

1/2 * 1/2 * 1/2 * 1/2 = 1/16 ... 4 half-lives ... 4 * 700 million = ?

alternatively ... (1/2)^n = 1/16 ... n log(1/2) = log(1/16)
... n = number of half-lives