Uranium-235, with a half-life of 713 million years, decays to lead-207. If you analyze a rock sample and find that it contains uranium-235 and lead-207 in a ratio of 1:3, how old is the rock?
a. 4600000000 years old
b.1426000000 years old
c.713000000 years old
d. 0 years old
e. 256500000 years old