Can someone help please? Copper occurs naturally as a mix of two isotopes: Copper-63 with a mass of 62.930 amu and copper-65 with a mass of 64.928 amu. The percent abundance of copper-63 is 69.09%.
Calculate the atomic mass of copper.

63.55

Explanation:

The attached chart shows the calculation.  The orange line points to the % abundance of Cu-65, which is the difference of 100% less the % Abundance of Cu-65 (since there are only 2 isotopes).

The weighted average contribution of each istope is calculated (blue arrow) and then summed to find the atomic mass of copper.

The answer to this problem is. 67

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.