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?

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?

a. The rock is approximately ? billion years old.

(Round to one decimal place as needed.)

b. The rock is approximately ? billion years old.

(Round to one decimal place as needed.)