having trouble with B.

Acertain element has a half life of 3.5 billion years. a. You nd a rock containing a mixture of the element and lead. You determine that 60% of the original element remains; the other 40% decayed into lead. How old is the rock? b. Analysis of another rock shows that it. contains 5% of its original element; the other 95% decayed into lead. How old is the rock? a. The rock is approximately 2.6 billion years old. (Round to one decimal place as needed.) in. The rock is approximately D billion years old. (Round to one decimal place as needed.)