In a class of 12 pupils, 9 have a brother and 7 have a sister, while 2 have neither. Using the addition theorem, we can deduce that 6 pupils have both a brother and a sister, and 10 pupils have either a brother or sister. It is important to note that the total number of pupils in the class is n(E) = 12, with n(B) = 9 having a brother, n(S) = 7 having a sister, and n(neither B nor S) = 2 having neither.