A certain element has a half life of 4.5 billion years.
a. You find a rock containing a mixture of the element and lead. You determine that ​15% of the original element​ remains; the other ​85% decayed into lead. How old is the​ rock? (round to one decimal place)
b. Analysis of another rock shows that it contains ​40% of its original​ element; the other ​60% decayed into lead. How old is the​ rock? (round to one decimal place)