The probability of event B is P(B) = 1/2
From the Venn diagram, we can observe that there are two events A and event B
n(A ∪ B) = 6 + 6 + 20
n(A ∪ B) = 32
And n(A ∩ B) = 6
The sample space would be,
n(S) = 6 + 6 + 20 + 20
n(S) = 12 + 40
n(S) = 52
And the number of outcomes for event B would be,
n(B) = 20 + 6
n(B) = 26
Using the definition of probability,
P(B) = n(B)/n(S)
P(B) = 26/52
P(B) = 1/2
This is the required probability.