Calculation of istopic abundance: The atomic mass of gallium `(Ga)` is 69.72 u. The masses of the naturally occuring istopes are `68.9257 u` for `._(69)^(31)Ga` and `70.9249 u` for `._(71)^(31) Ga`. Calculate the present adundance of each istope.
Strategy : We present the fraction of each istope algebraically. The atomic mass of an element is the weighted average of the istope masses, which equals the sum of the masses of each isotope times the fraction of that isotope.

Explanation

Let `X` = Fraction of `._(71)^(31)Ga`
Thus,
`X(68.9257 u) + (1 -X) (70.9249 u) = 69.72 u`
`68.9257X + 70.9249 - 70.9249X = 69.72`
`-1.9992 X = -1.20`
`X = 0.600`
Thus `._(69)^(31)G` is `60.0%` while `._(71)^(31)G` is `40.0%`