What is the percentage of $X_8^{18}$ for the isotopes $X_8^{16}$  and for $X_8^{18}$  with an average atomic mass of 16.2 u ?

The atomic mass of element X = 16.2 u. 
And the percentage of the isotope $X_8^{16}$=z
The percentage of the isotope $X_8^{18}$ $=100-z$
$\Rightarrow \frac{16z+18\left(100-z\right)}{100}$
Atomic mass of  X will be,
$16.2=\frac{16z+18\left(100-z\right)}{100}$
$1620=16z+1800-18z$
$2z=180$
$\Rightarrow Z=\frac{180}{2}$
$\Rightarrow Z=90$
As a result the percentage of the isotope $X_8^{16}=90\%$
And the Percentage of the isotope $X_8^{18}=100-90$
$\Rightarrow 10\%$
 Therefore, the percentage of isotope for $X_8^{18}$ is 10%.