Analysis of a rock sample shows that it contains 6.25% of its original uranium-235. How old is the rock? How do you know?

The rock is 2100 million years old by computing its half life.

Explanation:

The half life of Uranium 235 is 700 million years. it is an extremely radioactive component and produces a large amount of nuclear energy in reactions.

Half life of a product means that it will decompose into half of what it was in the time that is written so we can see that the sample would be halved in 700 million years of time.

The rock now retains only 6.25% of its original mass which means that it is 1/16th of what it used to be.

So, if the half life of the product makes the product out to be 1/2 in 700 million years

It means that to get to 6.25 it would require 3 more half lives which results that

3 times 700 million years

2100 million years.