The average atomic mass of a sample of element X is 16.1 u. What are the percentages of isotopes of  ¹⁸X₈ in the sample if the isotopes are ¹⁶X₈ and  ¹⁸X₈?

The percentages of isotopes of  ¹⁸X₈ in the sample is $5\%.$
The average atomic mass of the sample of element $X=16.1\mathit{ u}$. 

Let the percentage of isotopes ¹⁸X₈ be $\mathit{y }\%.$ Thus, the percentage of isotope ¹⁶E₈ will be ($100-y) \%.$

Therefore, 
$\frac{18\mathrm{ }\times \mathrm{y}}{100}+\mathrm{ }\frac{16\mathrm{ }\times \mathrm{ }(100-\mathrm{y})}{100}\mathrm{ }=\mathrm{ }16.1$

$\frac{(18y\mathit{ }+ 1600 - 16y)}{100} = 16.1$
$18\mathit{y }+ 1600 - 16\mathit{y }= 1610$
$2\mathit{y }+ 1600 = 1610$
$2\mathit{y }= 10$
Y = 5