A: (91.1*28 + 7.9*29+30)/100 = 28.009
B: (92.2*28 + 4.7*29 + 3.1*30)/100 = 28.109
So, B.
The idea is to multiply the relative abundance with the isotopic mass and take the average of the abundances.
Do note, though, that sometimes the three or four or however many abundance percentages are given DO NOT add up to 100%. You've to be careful what you divide with!
I know how to do it if i were to calculate the answers one by one, but i was actually looking for an alternative way to find it without doing that.
But i guess it's impossible since there's 3 unknown and 1 equation.
Thanks btw!