Suppose you find a rock that contains some potassium-40 (half-life of 1.3 billion years). you measure the amount and determine that there are 5 grams of potassium-40 in the rock. by measuring the amount of its decay product (argon-40) present in the rock, you realize that there must have been 40 grams of potassium-40 when the rock solidified. how old is the rock? a) 1.3 billion years b) 2.6 billion years c) 3.9 billion years d) 5.2 billion years e) none of the above