Counting & Probability · Challenge problem

Each of $100$ people owns at least one of a cat, a dog and a fish. Exactly $45$ own a cat, $50$ own a dog and $40$ own a fish. If you add the number of people owning both a cat and a dog, the number owning both a cat and a fish, and the number owning both a dog and a fish, the total is $40$. How many of the people own exactly one kind of pet?