Suppose You Find a Rock That Contains 10 Micrograms of Radioactive

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) 1) 25 billion years
B) 5) 0 billion years
C) 2) 5 billion years
D) 3) 75 billion years