Question

A rock sample contains 4.5g of Potassium-40 and 31.5g of its daughter isotope (Argon-40). How old is the rock sample if Potassium-40 has a half-life of 1.3 billion years?

Answer 1

I'm pretty sure it would be 3.9. Because you add 4.5 and 31.5 to get the starting amount of potassium-40. Then you divide that by two until you get 4.5. You count how many times you divided 2 from the number. (3 times) Then you multiply that by 1.3 and get 3.9 billion years. 

Answer 2

I was doing a test with this question and it said that 3.9 billion years was wrong.