A radio isotope X with a half life 1.4 × 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
1.92 × 109 years
2
3.92× 109 years
3
4.20 × 109 years
4
8.40 × 109 years
5
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