9+6= 15 (piece)

2. How many jiaozi did Mom eat more than Xiao Gang?

9-6=3 (pieces)

3. How many times does Mom eat jiaozi as Xiao Gang eats jiaozi?

9/6= 1.5 times

4. What percentage of jiaozi Xiao Gang ate accounted for the number of jiaozi his mother ate?

6/9=2/3