Answer:
There is no enough information to determine how many or what fraction of balloons Erin kept for herself.
We don't know how many balloons Erin bought in the first place, and we don't know what fraction of the balloon she gave to Karen. Suppose she gave 1/2 of the balloon to Karen, then she has 1/2 left. Giving 1/2 of what is remaining to Tim, she give 1/2 of 1/2 to her, which is 1/4. What is left is 1/4.
Therefore she keeps 1/4 for herself.
