Question

I find that there are 500 atoms of U-235 and 1500 atoms of Pb-207. If we assume the mineral contained only parent and no daughter, then how many atoms of these elements were in the mineral when the rock first formed?

Answer 1

Explanation:

From the question it can be infered that initially only parent atoms were present in the rock and  no daughter atom. So, initially there was only uranium atom and no lead atom. So, in total there were 500+1500= 2000 uranium atom. And no lead tom was there.