Question

A rock has 50 percent of its original amount of potassium-40 remaining in it. Potassium-40 has a half-life of 1.25 billion years. How long ago was the rock formed?

1.25 billion years ago

2.50 billion years ago

5.00 billion years ago

6.25 billion years ago

Answer 1

Half-Life Definition:
The time required to convert one half of a reactant to product. The term is commonly applied to radioactive decay, where the reactant is the parent isotope and the product is a daughter isotope.

The answer is in the same question, is like asking "what color is Peter's white horse?"