Actually, this little bit tricky question ( I think can't apply directly inclusion exclusion principal).
Let's look on the question what is given:
S--> Singapore
N-->New York
L--> London
n(S) = 22n,
n(N) = 28,
n(L) = 24,
n(S AND N BUT NOT L) = 6
n(S AND L BUT NOT N) = 4
n(N AND L BUT NOT S) = 8
n( N U S U L) = 42
According to question formula would be like that
n(S U N U N) = n( alone S) + n( alone N) + n( alone L) + n(S AND N BUT NOT L) + n(S AND L BUT NOT N) + n(N AND L BUT NOT S) + n(S AND L AND N)
Let's assume n(S AND L AND N) = x
42 = { 22 - ( 8 + 4 + x)} + { 28 - ( 8 + 6 + x)} + { 24 - ( 4 + 8 + x) } + { 8 + 6 + 4 } + x
2x = 56 - 42
X = 7
So we wanted to calculate
n(alone N) = 28 - { 8 + 6 + x} = 7
I hope it will help you to understand.