At my office, 17 people own cats, 9 people own dogs, and 3 people own both cats and dogs. How many people own a cat or a dog?
Draw a venn diagram
(this is more of a boxed diagram as I am unable to upload images with my current browser)
cats dogs
|-----------------------|----------------------------|
| | | |
| | | |
| 14 | 3 | 6 |
| | | |
|-----------------------|----------------------------|
Thus there are 14 + 3 + 6 = 23 people who have a cat OR dog.