A radioactive substance has a half-life of 5 million years. What is the age of a rock in which 25% of the original radioactive atoms remain?

Choose one answer.

a. 2.5 million years

b. 5 million years

c. 15 million years

d. 10 million years

Correct answer is D.

After each 5 million years the percentage of the original radioactive atoms halves.

After 5 million years 50% of the original radioactive atoms remains.

After 10 million years 25% of the original radioactive atoms remains.