Question

A rock contains a radioactive isotope that decays to half of its original

amount in 5 million years. When this rock formed, it was made up of 100%
of the radioactive isotope, and 0% of the new element that it decays
If you find this rock and analyze it, and find that it contains only 50% of the
radioactive isotope and 50% of the new element, how old must this rock
be? *

1) 2.5 millon years

2) 10 million years

3) 5 million years

4) 15 million years

Answer 1

3) 5 million years

Step-by-step explanation:

The rock is 5 million years old because the rock contains only 50% of the radioactive isotope. We know that this rock decays to become half in 5 million years. Initially, the rock has 100% of radioactive isotope and 0% of the new element that forms with the decaying of the rock. But 50% of the radioactive isotope decays and 50% of the new element is formed so we can say that it reaches to its half age which is 5 million years.