You find a rock containing a mixture of the element and lead. You determine that 5 5% of the original element remains; the other 95 95% decayed into lead. How old is the rock? b. Analysis of another rock shows that it contains 30 30% of its original element; the other 70 70% decayed into lead. How old is the rock? a. The rock is approximately nothing billion years old. (Round to one decimal place as needed.) b. The rock is approximately nothing