Answer:
Average atomic mass = 63.3896
Step-by-step explanation:
Step 1: Find how much Cu-65 we have
1 - Amount of Cu-63 = Amount of Cu-65
1 - 0.6915 = Amount of Cu-65
Amount of Cu-65 = 0.305
Step 2: Find the average atomic mass of Cu
(0.6915 x 63) + (0.305 x 65) = Average atomic mass
(43.5645) + (19.825) = Average atomic mass
63.3895 = Average atomic mass
Therefore the average atomic mass of Cu is 63.3895 atomic mass units