At my office, 16 people own cats, 11 people own dogs, and 7 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".)
By the principle of inclusion-exclusion(see here: https://brilliant.org/wiki/principle-of-inclusion-and-exclusion-pie/) We have that the answer to be $16 + 11 - 7 = 20$