(4 points) Radiometric dating. One radioactive process used to determine the age of rocks on Earth is the decay chain of uranium-238 to lead-206. The half-life for this decay chain is dominated by the slowest reaction, the initial alpha decay of uranium-238 into thorium-234: 238U234 Th + He with a half-life of 4.5 billion years. Fortunately the amount of another isotope, lead-204, in the initial sample is the same as observed today, because no lead-204 is produced by radioactive decay. Therefore, taking the ratio of lead-206 to lead-204 in nonradioactive rocks to be about 16:1, the same proportion can be assumed at the time of rock solidification. If the ratio of uranium-238 to lead-206 to lead-204 is now measured to be 14:20:1 in the sample of interest, what is the age of this rock?