In a 100g sample of copper, 69 g were Cu-63 while 31g were Cu-65. Find the average atomic mass.
To obtain an exact number you need the masses of each isotope (they won't be 63 and 65). In the absence of that, however, you substitute those numbers for the 63 and 65 in my answer and you will obtain the correct atomic mass.
(0.69*63) + (0.31*65) = ?