Question

The half life of Uranium 238 to lead 206 takes 4.46 billion years. Analysis of a hypothetical rock showed that there is 12.5 % Uranium present in the sample. How old is this rock in billions of years old? 

Answer 1

Sample = 100% $\to$ 87.5%$\to$ 75%$\to$ 62.5%$\to$50%$\to$ 37.5%$\to$ 25%$\to$ 12.5%

no. of half life (n) = 8

t_1/2 ( time of half life )

How old ( in time ) = n × t_1/2

= 8× 4.46 billion

= 35.68 billion years