Question

If 6.25% of a parent radioactive atoms remain in a rock, and the half life is 200,00 years old how old is the rock?

Answer 1

800,000 years.

Explanation:

6.25 = 1/16 or `1/2^4`

This means that the rocks has experienced 4 half lives of 200,000 years.

`4 xx 200,000 = 800,000`

I do not know of any radioactive matter with a half life of 200,00 years.