Just ran the test and seems to have gotten it correct.
Okay, let's think through this step-by-step:
We know that Sally is a girl and she has 3 brothers.
Each of Sally's brothers has 2 sisters.
Now, who are these sisters? They must be Sally and one other sister, because if Sally's brothers had any other sisters, they would also be Sally's sisters, and the problem doesn't mention that.
So, if each of Sally's brothers has 2 sisters, and one of these sisters is Sally herself, then Sally must have 1 other sister.
Therefore, Sally has 1 sister.
Okay, let's think through this step-by-step:
We know that Sally is a girl and she has 3 brothers. Each of Sally's brothers has 2 sisters. Now, who are these sisters? They must be Sally and one other sister, because if Sally's brothers had any other sisters, they would also be Sally's sisters, and the problem doesn't mention that. So, if each of Sally's brothers has 2 sisters, and one of these sisters is Sally herself, then Sally must have 1 other sister. Therefore, Sally has 1 sister.