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## Homework Statement

A rock contains three

^{207}Pb nuclei for every

^{235}U nucleus. Determine the age of the rock assuming all of the

^{207}Pb was formed through the decay of uranium. (The half life of

^{235}U is 0.71x10

^{9}).

## Homework Equations

N=N

_{0}e

^{-λt}

## The Attempt at a Solution

So the decay constant is ln 2/T

_{1/2}=9.76x10

^{-10}. So

N

_{0}= 1 e

^{(9.76x10-10)t}

But the problem is I don't know how to find N

_{0}, the remaining nuclei. How could I find it using the information given? They just say there are 3 Pb's for every uranium nucleus... How do we find the amount decayed so we can subtract it from the total to get the remaining N

_{0}?