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`.

Explanation

Since the number of `.^(205)Pb` atoms equals the number of `.^(238)U` atoms, half of the original `.^(238)U` atoms have decayed. It takes one half-life to decay half of the active nuclei. Thus, the sample is `4.5xx10^(9)y` old.