Probability of getting a king is 4C1 / (52) [ because there are total 4 Kings in a deck ]
=> P(king) = 4/52
Probability of getting a diamond card
= 13C1/52 [ because there are 13 cards of a suit]
=> P( diamond ) = 13/52
Probability of getting a diamond and king
= 1C1/52 [ because out of 13 diamond cards only 1 is king ]
=> P(diamond and king) = 1/52
P( A U B ) = P(A) + P(B) - P(A & B) where '&' denotes intersection symbol.
Now P ( diamond or king ) = P(diamond) + P(king) - P(diamond and king)
=> P(diamond or king ) = 13/52 + 4/52 - 1/52
=> P(diamond or king ) = 16/52 = 4/13
