A potassium-40 sample starts with 50 atoms, and 1.25 billion years later, there are 25 atoms. What is the half life of potassium-40?

A. 25 years

B. 40 years

C. 1.25 years

D. 2.5 years

Answer;

Potassium-40 has a half life of 1.25 billion years.

Explanation;

New amount = Original amount * (1/2) ^n; where n is the number of half lives.

Original amount is 50 atoms

New amount is 25 atoms

Therefore;

25 = 50 * (1/2) ^n

1/2 = (1/2) ^n

Thus; n = 1

n = 1 which is equivalent to 1.25 billion years

Therefore;

Potassium-40 has a half life of 1.25 billion years.