Of an initial amount of 5000 g of lead-210, how much will remain in 190 years? Lead-210 decays at a rate of 3.15%/yr. g (Round to one decimal place as needed.) 6 Uranium-238 (U-238) has a half-life of 4.5 billion years. Geologists find a rock containing a mixture of U-238 and lead, and determine that 58% of the original U-238 remains; the other 42% has decayed into lead. How old is the rock? The rock is billion years old. (Round to three decimal places as needed.) 8