Naturally occurring gallium consists of 60.108% Ga-69, with a mass of 68.9256 AMU, and 39.892% Ga-71, with a mass of 70.9247 AMU. The average atomic mass of gallium will be ____

59 A.M.U

69 A.M.U

79 A.M.U

89 A.M.U

Similarly, you could input the percentages normally and then divide by $100$ :

$M_{r} (Ga) = \frac{(68.9256 \cdot 60.108) + (70.9247 \cdot 39.892)}{100}$

$M_{r} (Ga) = 69.7231 to 4 decimal places$