How would you calculate the average atomic mass of copper if 69.2% of copper has a mass of 62.93 amu and 30.8% has a mass of 64.93 amu?
Simply put, each isotope will contribute to the average atomic mass of the element proportionally to its percent abundance.
#color(blue)("avg. atomic mass" = sum_i ("isotope"_i xx "abundance"_i)#
As far as the actual calculations go, you will use decimal abundances, which are simply percent abundances divided by
So, you know that the atomic masses of these two copper isotopes are
The average atomic mass of copper will thus be
#"avg. atomic mass" = overbrace("62.93 u" xx 0.692)^(color(red)(1^"st" "isotope")) + overbrace("64.93 u" xx 0.308)^(color(red)(2^"nd" "isotope"))#
#"avg. atomic mass " = color(green)(" 63.546 u")#
I'll leave the answer is rounded to four sig figs, despite the fact that the values you have for the percent abundances justify only three sig figs.