Question

suppose you found a material in which 12.5% of the original radioactive atoms were present. if the half-life is 47 years, how old is the material?

Answer 1

If it has 12.5% of the atoms remaining then it has undergone 3 half-lives. After the first, it would have 50%, the second 25%, the third 12.5%. So 3 half-lives, and each one is 47 years. You should be able to figure it out from there.