math
posted by eric .
Mrs. Bollo's second grade class of thirty students conducted a pet ownership survey. Results of the survey
indicate that 8 students own a cat, 15 students own a dog, and 5 students own both a cat and a dog. How
many of the students surveyed own no cats?

The answer's right in the question. How many do you see that do not have cats?

The question needs to be more clearly worded.
If those 8 owning a cat includes those that may also also own a dog, and those 15 owning a dog include those that might own a cat, then there are 15 + 8 5 = 18 with a dog or a cat or both. That leaves 12 with neither.
There is another way the question could be interpreted. The eight might own ONLY cats and the 15 might own only dogs, with an additional 5 owning both. Then you get a different answer. 
There are also students that own neither. The questions are not written well. Do you have an example of one the teacher did so we have something to work from to know what (s)he is asking?