What is the age of a meteorite if potassium-40 decayed from 80 g to 10 g? The half-life of potassium-40 is 1.3 billion years.

10 / 80 amount of potassium-40 remaining

1/8 = 1/2 * 1/2 * 1/2 = 3 half-lives

3 half-lives * 1.3E9 yrs / half-life = 3.9E9 yrrs

After 3.9 billion years the 80 g will have decayed to 10 g

the answer would be a

close lines strong magnet less close weaker