Question 380289
There are 15 who own dogs, so 30-15 or 15 do not own a dog.

Or, since there are 5 that own both, then 3 students own a cat (only), 10 own a dog (only) and 5 own both. It follows that 12 students have no pets (or own pets other than cats or dogs). Thus the number of people who don't have a dog is 3+12=15.