A rock sample containing an isotope with a half-life of 28 million years has an initial mass of 184 grams. How much time has elapsed after three half-lives?
A) 6.5 million years
B) 9.3 million years
C) 32 million years
D) 56 million years
E) 84 million years

Question

Quizplus · Accepted Answer

After three half-lives, the time elapsed is 3 times the half-life of the isotope. Since the half-life is 28 million years, the time elapsed is 3 * 28 million years = 84 million years.

A Rock Sample Containing an Isotope with a Half-Life of 28