In this case, we'll have to carry out several steps to find the solution.
Step 01:
Data
4 / 9 = boys
ratio of girls to boys = ?
Step 02:
Tape Diagram
boys = 4 / 9
girls = 5 / 9
girls to boys = 5 / 4
The answer is:
The ratio of girls to boys is 5 / 4