Question

The half-life of Uranium-238 is 4.5 billion years. A 100 gram sample of the Uranium-238 sits in a cave, untouched, for 9 billion years. How much of the sample is left.

Answer 1

25 grams

Explanation:

The half-life is 4.5 billion years. That means that after each period of 4.5 billion years, only half the sample remains. Nine billion years includes two such time periods.

Start: 100 grams

After 4.5 billion years, 50 grams remain.

After another 4.5 billion years, 25 grams remain.