There are 28 students in a class, so what is the possibility that three or more students will be born in the same month?

Can be pushed out

Twenty-eight students must have three or more students born in the same month, that is, the probability that three or more students were born in the same month is 100%.

This is pigeonhole principle.

But you can't say the opposite,

That is to say, b can be derived from a, but a may not be derived from B.

For the question:

How many students must there be in a class to ensure that at least four students are born in the same month?

This is a lack of conditions, or to add qualifications.

Only when the number of students in each month is the same, and the number of students in a class is the least, can at least four students be born in the same month.