I can get you started. When I read this problem I see the adults and children organized into small groups. Each group has 2 adults and 9 children. In each group the number of adults (2) is 2/9 the number of children ( 2/99 = 2).
In each group there are 7 more children than adults and hence there are 100 groups to give 700 more children than adults. Each group has 11 people in it so, in total, there are 1100 people at the zoo, 200 adults and 900 children.
Lets check my work so far.
2/9900 = 200 so the number of adults is 2/9 the number of children.
To return to the previous page use your browser's back button.