Exercise XI — task. Science CBSE, Class 9.

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The average atomic mass of a sample of an element X is $16.2$ u. What are the percentages of isotopes $_{8}\textrm{X}^{16}$ and $_{8}\textrm{X}^{18}$ in the sample?

$\begin{array}{l} (\frac{16 \times x}{100}) + \frac{18 (100 - x)}{100} = 16.2 \\ \frac{16 x - 18 x + 1800}{100} = 16.2 \\ - 2 x + 1800 = 16.2 \times 100 \end{array}$

$x = 10$ and $(100 - x) = 90$
$x = 80$ and $(100 - x) = 20$
$x = 20$ and $(100 - x) = 80$
$x = 90$ and $(100 - x) = 10$

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11. Exercise XI

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