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The number of ^238 U atoms in an ancient...

The number of `^238 U` atoms in an ancient rock equals the number of `^206 Pb` atoms. The half-life of decay of `^238 U` is `4.5 xx 10^9 y`. Estimate the age of the rock assuming that all the `^206 Pb` atoms are formed from the decay of `^238 U`.

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Since the number of `^206 Pb` atoms equals the number of `^238 U` atoms, half of the original `^238 U` atoms have decaayed. It takes one half-life to decay half of the active nuclei. Thus, the sample is `4.5 xx 10^9 y` old.
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