A sample contains 16 point 7 5 g of the radioisotope cap u - 236 and 50 point 2 5 g of its daughter isotope, Th - 232 How long did it take for decay to take place if one half-life of cap u - 236 is 23 million years?

The original amount of uranium-236 was the remaining uranium plus the daughter isotope: 16.75 g + 50.25 g = 67 g. The uranium remaining is 16.75/67 = 1/4 of the original amount, which means two half-lives have passed (67 g → 33.5 g → 16.75 g). At 23 million years per half-life, the elapsed time was 2 × 23 = 46 million years, choice D.

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