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An element is found in nature in two isotopic forms with mass numbers $(A-1)$ and $(A+3)$. If the average atomic mass of the element is found to be A, then the relative abundance of the heavier isotope in the nature will be

(A) $60 \%$

(B) $75 \%$

(C) $25 \%$

(D) $40 \%$

Solution —

Let relative abundance of heavier isotope be $x \%$.

$$\begin{array}{l} A=\frac{(100-x)(A-1)+x(A+3)}{100} \\ 100 A=100 A-x A-100+x+3 x+x A \\ 4 x=100 \\ \therefore x=25 \end{array}$$

Therefore, The correct option will be (C) 25 %

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