Suppose you find a rock and measure that 12.5% of the original uranium-235 still remains it, while the other 87.5% has decayed into lead-207. about how old is the rock?

Uranium-235 is an isotope of uranium that is fissionable and appears naturally. His half-life is 713x10^6 years (703 million years). Therefore, half of Uranium 235 decayed into Lead-207 in 1 half-life. Based on this, we know that in 1 half-life there will be 50% of Uranium-235 and 50% of Lead-207, in two half-lives there will be 25% of Uranium-235 and 75% of Lead-207, finally, in three half-lives there will be 12.5% of Uranium-235 and 87.5% of Lead-207. Then, we have:
 713x10^6 years x 3 half-lives = 2139x10^6 years= 2.139 billion years.
 
 How old is the rock?
 
 The answer is: 2.139 billion years