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    Can anyone tell me where to go with this question. It's only two marks but I have no idea where to start
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    Well at any time the number of Pb206 atoms plus the number of U238 atoms must equal the original number of U238 atoms because the question says there is nowhere else for Pb206 to come from other than the decay of U238.

    if at a particular instant you've got a Pb206238 ratio of 1:2 that means for every 100 atoms of Pb you've got 200 atoms of U238... and originally every Pb206 atom was a U238 atom - you must have started with 300 U238 atoms.
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    Ahhhh yes. Seems so simple now. I think the wording just threw me off
 
 
 
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