Naturally occurring gallium is a mixture of isotopes that contains 60.11% of Ga-69 (atomic mass = 68.93 u) and 39.89% of Ga-71 (atomic mass = 70.92 u). Which numerical setup can be used to determine the atomic mass of naturally occurring gallium?
You have to assume that you have 10000 atoms of gallium which would mean that 6011 of those atoms weigh 68.93 u while 3989 atoms weighs 70.92 u. So to calculate this calculation can be set up this way.
