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Half lives of two isotopes X and Y of a ...

Half lives of two isotopes X and Y of a material are known to be `2 xx 10^(9)` years and `4 xx 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

`2xx10^(9)` years

B

`4xx10^(9)` years

C

`6xx10^(9)` years

D

`8xx10^(9)` years

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The correct Answer is:
D
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