A Rock Contains 0 $\rarr$ Pb-206 Is 4

A rock contains 0.37 mg of Pb-206 and 0.95 mg of U-238. The half-life of the decay series U-238 $\rarr$ Pb-206 is 4.5 * 10⁹ yr. Assuming no Pb-206 was present in the rock initially, how old is the rock

A) 1.7 * 10⁹ yr
B) 5.2 * 10⁹ yr
C) 2.7 * 10⁶ yr
D) 4.5 * 10⁹ yr
E) 2.4 * 10⁹ yr