13.Nuclei
medium

A radio isotope $X$ with a half-life $1.4 \times 10^{9}\; years$ decays of $Y$ 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 ........ $ \times 10^9\; years$

A

$2.4$

B

$1.4$

C

$4.2$

D

$5.2$

(AIPMT-2014)

Solution

Here, $\frac{X}{Y}=\frac{1}{7}$

$\frac{X}{X+Y}=\frac{1}{1+7}=\frac{1}{8}=\frac{1}{2^{3}}=\left(\frac{1}{2}\right)^{3}$

$n=3$

$\frac{t}{T}=3$

$t=3 T=3 \times 1.4 \times 10^{9} years$

$t=4.20 \times 10^{9} years$

Standard 12
Physics

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