An ancient rock sample has 2 pg/kg of a radioactive isotope. Modern formations of this type of rock have 64 pg/kg of this same isotope. If the half-life of this isotope is 30 million years how old is the ancient rock sample

180 million years

Explanation:

Data provided in the question:

Rock sample has 2 pg/kg of a radioactive isotope

Isotope of the modern rock formed = 64 pg/kg

Half life of the isotope = 30 million years

Now,

From 128ug/kg to 2 ug/kg,

Number of half-lives elapsed = 6

Therefore,

The Age of the rock sample

= Half life of the isotope * Number of half-lives elapsed

= 30 million years * 6

= 180 million years