6 A certain element has a half life of 2.5 billion years. a. You find a rock containing a mixture of the element and lead. You determine that 40% of the original element remains; the other 60% decayed into lead. How old is the rock? b. Analysis of another rock shows that it contains 65% of its original element; the other 35% decayed into lead. How old is the rock? a. The rock is approximately billion years old. (Round to one decimal place as needed.)