The question meant that a cake is cut into 9 equal part.

Geraldine ate 2 of them so there are 7 pieces left.

Aldrich ate more than Xiling. And Alrich and Xiling ate 7 piece together,

The possibility is as follow
Alrich eat 4 and Xiling eat 3;
Alrich eat 5 and Xiling eat 2;
Alrich eat 6 and Xiling eat 1;

So at maximum Xiling can eat only 3.

Now what proportion of the original cake (9 pieces) does Xiling eat if Xiling ate 3 pieces?

Oooohhhh I understand already. Thank you teacher !!