Suppose you find a rock that contains 10 micrograms of radioactive potassium-40, which has a half-life of 1.25 billion years. By measuring the amount of its decay product (argon-40) present in the rock, you conclude that there must have been 80 micrograms of potassium-40 when the rock solidified. How old is the rock?
A) 3.75 billion years
B) 1.25 billion years
C) 2.5 billion years
D) 5.0 billion years

Question

Quizplus · Accepted Answer

Since the half-life of potassium-40 is 1.25 billion years, we know that half of the original 80 micrograms would decay to become 40 micrograms in 1.25 billion years. Therefore, the initial 80 micrograms would decay to become 40, then to 20, then to 10 over a period of three half-lives, which is a total of 3.75 billion years. Therefore, the rock is 3.75 billion years old.

Suppose You Find a Rock That Contains 10 Micrograms of Radioactive