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5. [4 marks] Radioactive decay: Natural uranium found in the Earth's crust contains the isotopes 92 235U and 92 235 U in the atomic ratio 7.3 x 10" to 1. Assuming that at the time of formation these two isotopes were produced equally; estimate the time since the formation given that the mean lifetimes of the two isotopes are 7235 = 1.03 x 10" years and T238 = 6.49 x 10" years

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