A school allowed the students of a class to go to swim during the days March $11th$ to March $15$, $2019$. The minimum number of students the class should have had that ensures that at least two of them went to swim on the same set of dates is?
I concluded that in the given $5$ days we can set to students. So to get the least number of students we can get $(5*2+1)=11$ but the answer is given $33$. Can anyone explain this problem?