Answer:
0.233 or

Explanation:
Pre-Solving
We are given that 12 people have a brother and a sister, 7 people have a sister but don't have a brother, 3 have a brother but don't have a sister, and 8 don't have either.
We want to find the probability that a random student has a sister but doesn't have a brother.
Recall that probability is

So, we have 7 favorable outcomes (7 people that have a sister but don't have a brother), and when we add the total number of people together, we get 30 (the total number of people surveyed, aka our total outcomes).
Solving
Divide 7 by 30.

We can simplify this:
0.233