A radio isotope 'X' with a half life 1.4 x 109 years decays to 'Y' which is stable. A sample of the rock from a cave was found to contain 'X' and 'Y' in the ratio 1:7. The age of the rock is :
1.96 x 109 years
3.92 x 109 years
4.20 x 109 years
8.40 x 109 years
Since Y forms due to decay of X.
at t =t, suppose number of X=N
then number of Y =7N
Total number =8N =No.
Option 1)
1.96 x 109 years
This option is incorrect.
Option 2)
3.92 x 109 years
This option is incorrect.
Option 3)
4.20 x 109 years
This option is correct.
Option 4)
8.40 x 109 years
This option is incorrect.