Logic Question.

For me we are missing one equation here. We don't know the intersection of B and C in order to say how many people are outside of all groups.

We have 8 variables:
-people on A
-people on B
-people on C
-people on AB
-people on AC
-people on BC
-people on ABC
-people on none

And 7 equations:
A+AB+AC+ABC = 49
B+AB+BC+ABC = 55
C+AC+BC+ABC = 43
AB+ABC = 18
AC+ABC = 28
ABC = 8
A+B+C+AB+AC+BC+ABC+none = 100