` Half lives of two isotopes X and Y of a material are known to be \[2\times {{10}^{9}}years\] and \[4\times {{10}^{9}}years\,\]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. \[2\times {{10}^{9}}years\]
• B. \[4\times {{10}^{9}}years\,\]
• C. \[6\times {{10}^{9}}years\]
• D. \[8\times {{10}^{9}}years\]