+173 A poll of 100 eighth-grade students was conducted to determine the number of students who had a dog, a cat, or a fish. The data showed that 50 students had a dog, 40 students had a cat, and 20 students had a fish. Further, 19 students had only a dog and a cat, 2 students had only a cat and a fish, 3 students had only a dog and a fish, and 12 students had only a fish. How many students had none of these pets?
