t my office, 16 people own cats, $9$ people own dogs, and $5$ people own both cats and dogs. How many people own a cat or a dog?
(Remember that in mathematics, the word "or" is used in the inclusive sense. So if you own both a cat and a dog, then you are included in the category of people "who own a cat or a dog".)
Take the number of people who own cats.
Add to this, the number of people who own dogs.
From this total, subtract the number of people who own both cats and dogs.