Answer:
Three (3) half-lives have occurred since the sample of radioactive potassium started to decay
Explanation:
Argon to potassium ratio after 1 half life = 1:1
After 2 half lives = 75/25
= 3:1
After 3 half lives = 87.5/12.5
= 7:1
Since the ratio of the number of argon to potassium atoms is found to be 7:1, then Three (3) half-lives have occurred
