The isotope potassium-40 (40K) decays into the isotope argon-40 (40Ar) with a half-life of 1.25 billion years. Suppose we find a rock in which seven-eighths of the potassium-40 has decayed to become argon-40, and only one-eighth of the original potassium-40 remains.

Answer 1

1/8 = (1/2)^3
so, 3 half-lives have passed.

Answer 2

2.6 billion years

Answer 3

69 hahahahaha

Answer 4

69.420 billion years lol ()