The average atomic mass of an element is 16.2u. The percentage of isotopes ¹⁶C and ¹⁸C in the sample is 90% and 10%.

Let the percentage of isotopes ¹⁶C = x and ¹⁸C =(100-x)

Average atomic mass of Carbon $= \frac{16\times x}{100}+\frac{18(100-x)}{100}$
$= \frac{1800-2x}{100}$
$= 18-0.02x$
$18-0.02x=16.2$

 $x = \frac{1.8}{0.02}$
$\Rightarrow x= 90$
The percentage of isotopes ¹⁶C = 90%
The percentage of isotopes ¹⁸C $= (100-90)$
$\Rightarrow 10\%$.

Therefore, The percentage of isotopes ¹⁶C and ¹⁸C in the sample is respectively is 90% and 10%.