If a rock was determined to be around 3.9 billion years old how much Potassium 40 would be left in that rock sample? (Percentage of Potassium 40)

What percent of potassium-40 remains undecayed in a rock that is 3.9 billion years old? The 3.9 billion year old rock has undergone three half-lives (3.9 divided by 1.3 = 3). After 3 half-lives, 12.5% of the potassium-40 remains undecayed.

answer: "circumcision is a common procedure in which the skin covering the tip of the penis is surgically removed."

thinking it would be d not too sure but i think it's right