Each group has the same number of people in it.
So A=B and A+B=100%
The 9% of the TOTAL consists of 2 % of the first half (B), and 16% of the second half of the group (A).
2% of B, is actually only 1% of the total (A+B).
16% of A, is actually only 8% of the total (A+B).
Together they are 1+8=9% of the total.
of it, 1% belongs to B.
I hope i manged to explain it