A tour group. . .

Since the topic is random, there is no limit to where everyone can go. If there are many people in one or two places, there will be many people visiting the same place in the rest, so try to divide it equally to get the least.

Think of A, B and C as three drawers.

If you put 50 people in three drawers, the most uniform way is 16 people, 16 people, 17 people.

Anyway, there is always a drawer with more than 17 people.

Therefore, at least 17 people visited the same place.

Note: This is the content of the math contest-the pigeon hole principle. I just need to know, but I should not pass the exam. Of course, it's good to look at relevant knowledge if you are interested.