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The half life of a radioactive isotope 'X' is 20 years. It decays to another element 'Y' which is stable. The two elements ‘X’ and 'Y' were found to be in the ratio 1 : 7 in a sample of a given rock. The age of the rock is estimated to be

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100 years

60 years

40 years

80 years

answer is D.

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Detailed Solution

At present, parent to daughter nucleus are in the ratio 1 : 7.
It means parent is $\frac{1}{8}$
And daughter is $\frac{7}{8}$
So, parent nucleus becomes 1/8th of original.
So, age of the rock is = 3 x 20 = 60 years.

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