sets

[demoskopico]

Among the 20 students in the group, 5 study math, 10 study physics, and 6 study chemistry. Are there any students who do not study any of the above-mentioned subjects?

1.There are no students studying all of the three subjects
2.None of those who study math study chemistry

is the formula for sets with 3 subset: a+b+c - (double sets) - 2*(triple sets)+ not belonging to any set?

[kevin]

yes. that's right!

[demoskopico]

yes. that's right!

in other cases I see the formula must be : a+b+c-double sets + 1 triple set + n. can you tell me why and what is the difference?