# NEET Physics Nuclei Questions Solved

A radioactive isotope X with a half-life of $1.37×{10}^{9}$ years decays to Y which is stable. A sample of rock from the moon was found to contain both the elements X and Y which were in the ratio of 1 : 7. The age of the rock is
(a) $1.96×{10}^{8}$ years         (b) $3.85×{10}^{9}$ years
(c) $4.11×{10}^{9}$years          (d) $9.59×{10}^{9}$ years

(c) If in the rock there is no Y element, then the time taken by element X to reduce to $\frac{1}{8}\mathrm{th}$ the initial value will be equal to $\frac{1}{8}={\left(\frac{1}{2}\right)}^{\mathrm{n}}$or n =3
Therefore, from the beginning three half life time is spent. Hence the age of the rock is

.

Difficulty Level:

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