a rock sample contains 25 percent k-40 and 75 percent daughter product ar-40. if k-40 has a half-life of 1.3 billion years, how old is the rock?

2.6 billion years old

Explanation:

Initial:

100% k-40

0% ar-40

After 1.3 billion years:

50% k-40

50% ar-40

After 2.6 billion years:

25% k-40

75% ar-40

So the rock is 2.6 billion years old.