A   B   C
--+-------------
1 | x   a   1
2 | x   b   0
3 | y   a   1
4 | x   b   1
5 | y   a   1

Filter A: x

            a           b           total
--------+-----------------------------
        |   #   %       #   %       #   %  (the exact content of the row not matter here)
--------+-----------------------------
count x |   1   -       2   -       3   -
sum b   |   1   100%    1   50%     2   67%