If the half-life of a material is 2000 years and you found a rock layer that contained equal amounts of this material and its daughter material, the rock layer is about ________________________-.
A. 1000 years old
B. 2000 years old
C. 20,000 years old
D. 500 years old

Question

If the half-life of a material is 2000 years and you found a rock layer that contained equal amounts of this material and its daughter material, the rock layer is about ________________________-.
	A.	1000 years old
	B.	2000 years old
	C.	20,000 years old
	D.	500 years old

anonymous · Answer

B. 2000 years old. 
When 1/2 of it is gone it has decomposed into its daughter element and there is therefore half of the daughter element and half of the original, therefore 2000 years went by. 
A is wrong as then it would be 75% original and 25% daughter
C is wrong as there would be 3.125% original and 96.875% daughter
D is wrong as there would be ~80% original and ~20% daughter