500
BONUS (not as hard on test):
A sample is found to contain 20g Argon-40, and 60g of Potassium-40. Assuming that Potassium-40 decays into Argon-40 with a half-life of 1.3 billion years...how old is the sample?
What is 650 million.
1/2 of a half life has occurred (20+60 = 80g of Potassium-40 originally --> if 1 half life occurred, then 40g would be Argon-40 but only 20g has formed, therefore, 1/2 of a half life = 1/2 of 1.3billion)