Count a class with sets

Python's sets do the counting of a Venn diagram for you. A class has the roll numbers $1$ to $40$. The students whose roll number is even take mathematics; those whose roll number is a multiple of $3$ take physics.

The idea. & gives the intersection, | the union and - the difference of two sets.

Your tasks.

1. Set both to the students who take both subjects.
2. Set neither to the number of students who take neither.
3. Set only_maths to the students who take mathematics but not physics.