When two dice are rolled,
The possible outcomes for a total of 7,
A = {(1,6), (2,5) , (3,4), (4,3), (5,2), (6,1)}
The possible outcomes for a total of 11,
B= {(5,6), (6,5)}
And the above two sets are disjoint.
==> A U B = {(1,6), (2,5) , (3,4), (4,3), (5,2), (5,6), (6,1), (6,5)}
So Number of rolls results in a total of 7 or 11 = n( A U B) = 8