Suppose the percentages of \(^{16}_8X\) and \(^{18}_8X\) are Y and 100 - Y .
∴ Average atomic mass = \(\frac{Y}{100}\, \times\,16\,+\,\frac{100\,-\,Y}{100}\,\times\,18\)
16.2 = \(\frac{8Y\,+\,9(100\,-\,Y)}{50}\)
8Y + 900 - 9Y =16.2 x 50
Y = 900 - 810
Y = 90
∴ The percentage of first isotope is 90 and percentage of second isotope is 10.