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 35% of the original element remains; the other 65% decayed into lead. How old is the rock? (b). Analysis of another rock shows that it contains 55% of it's original element; the other 45% decayed into lead. How old is the rock.

A ) 50% is gone so 4.5 billion right of the bat. Then another 4.5 if it got down to 25% but it’s still at 35%. So it lost 15% from 50%. 15/25 * 4.5 billion = 2.7 billion +the original 4.5 = 7.2 billion years old.

B) it loses 45%. 50% would be 4.5 billion. 45/50 * 4.5 billion = 4.05 billion years old