JEE PHYSICS MCQ: NUCLEI

In the chemical analysis of a rock the mass ratio of two radioactive isotopes is found to be $100: 1$. The mean lives of the two isotopes are $4 \times 10^9 {yr}$ and $2 \times 10^9 \mathrm{yr}$, respectively. If it is assumed that at the time of formation the atoms of both the isotopes were in equal proportion, the age of the rock $p \times 10^{10} \mathrm{yr}$, where the value of $p$ is ......... (Take, ratio of the atomic weights of the two isotopes is $1.02: 1$.)