Final answer:
To be certain that at least two people have the same number of cats, we can use the Pigeonhole Principle.
Step-by-step explanation:
To be certain that at least two people have the same number of cats, we can use the Pigeonhole Principle. There are 52 cat owners at the convention, including the Crazy Cat Lady who owns 50 cats. The maximum number of different numbers of cats that the remaining 51 people can own is 51. Since there are more people than different numbers of cats, there must be at least two people who have the same number of cats.