Question

Uranium-lead (U-Pb) dating of geological samples is one of the oldest and most refined radiometric dating methods, able to determine ages of about 1 million years to over 4.5 billion years with precision in the 0.1–1% range. The U-Pb dating method relies on two separate decay chains, one of which is the uranium series from 238U to 206Pb, with a half-life of 4.47 billion years.

Geologists unearth a sample of zircon that appears to be a closed system. They find 0.766 microgram of ²⁰⁶Pb for 1.000 microgram of ²³⁸U present. Approximately how old is the sample?