Half lives of two isotopes X and Y of a material are known to be `2xx10^(9)` years and `4xx10^(9)` years respectively if a planet was formed with equal number of these isotopes, then the current age of planet, given that currently the material has `20%` of X and `80%` of Y by number, will be:
A. `2xx10^(9)` years
B. `4xx10^(9)` years
C. `6xx10^(9)` years
D. `8xx10^(9)` years
