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
A) 3.75 billion years
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