If an isotope has a half-life of 50 million years and an 1/8 of the sample remaining consists of that isotope, the age of the rock from which it was taken is:
A) 400 million years.
B) 300 million years.
C) 150 million years.
D) 800 million years.
E) none of the above

Question

Quizplus · Accepted Answer

The age of the rock is 150 million years because 1/8 of the sample remaining means that 7/8 of the sample has decayed. Since the half-life is 50 million years, 7 half-lives have passed, which is 7 * 50 million years = 350 million years. Subtracting the age of the rock from the total time elapsed gives 350 million years - 200 million years = 150 million years.

If an Isotope Has a Half-Life of 50 Million Years