Question

Scientists found stone tool specimens in a cave in Africa. Using potassium–argon radiometric dating, they discovered that the tools contained 25 percent radioactive potassium-40. How old are the tools? (You must show your calculations to receive credit.) *The half-life for potassium-40 to argon-40 is 1.25 billion years.

Answer 1

The potassium remaining is 1/4 of the original, so two half lives have passed.
One half life = 1.25 billion years
Two half lives = 1.25 x 2 = 2.5 billion years

The tools are 2.5 billion years old.