解答は以下になります。
n(U) = 100
n(A) = 80
n(B) = 45
n(C) = 40
n(A∩B) = x
n(B∩C) = 20
n(C∩A) = 25
n(A∩B∩C) = 8
n(A∩B) = n(A) + n(B) - n(A∪B) …①
n(A∪B) = n(U) - n(C) + n(B∩C) + n(C∩A)
- n(A∩B∩C) …②
①②より
n(A∩B) = n(A) + n(B) - {n(U) - n(C) + n(B∩C)
+ n(C∩A) - n(A∩B∩C)}
= n(A) + n(B) + n(C) -n(U)
- n(B∩C) - n(C∩A) + n(A∪B∩C)
= 80+45+40-100-20-25+8
=28