Several different radioisotopes can be used much like carbon-14 for determining the age of rocks and other matter. In the geosciences, the age of rocks may be determined using potassium-40, which decays to argon-40 with a half-life of 1.20 billion years.
1.) Some rocks brought back from the moon were dated as being 3.60 billion years old. What percentage of the potassium-40 has decayed after that time?
Express your answer with the appropriate units.
2.) Give a reason why uranium-238 (half-life of 4.6 billion years) is more useful for confirming this age than is carbon-14.