The method you need to use is explained at
http://geology.about.com/od/geotime_dating/a/uraniumlead.htm
A crystal of zircon separated from an igneous rock, was analyzed, and found to contain 7930 atoms of 207Pb, and 12070 atoms of 235U. Knowing the half life of 235U is 713 million years, how old is the zircon crystal? Please show your calculations so that I can figure out how to do this type of problem.
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