Let the % of isotope \(_{8}^{16}{X}\) = x and % of isotope \(_{8}^{18}{X}\) = 100 -x

Average atomic mass of an element (X) = 16.2

Average atomic mass = Mass of \(_{8}^{16}{X}\) \(\times\) percentage of \(_{8}^{16}{X}\)+ \(\frac{Mass\;of\; _{8}^{18}{X} \times percentage\;of\;_{8}^{18}{X}}{100}\)

or 16.2 = \(\frac{x \times+(100-x)\times18}{100}\)

or 16.2 = \(\frac{16x+1800-18x}{100}\)

or 1620 = 16x + 1800 - 18x

or 1620 = -2x + 1800 or 2x = 1800 - 1620

or 2x = 180

∴ x = \(\frac{180}{2}\) = 90

Thus % of isotope \(_{8}^{16}{X}\) = 90%

% of isotope \(_{8}^{18}{X}\) = (100 - 90) = 10%