Consider the example of finding the probability of selecting a black card or a 6 from a deck of 52 cards.
5/9
6/7
5/13
7/13
Explanation:
We need to find out P(B or 6)
Probability of selecting a black card = 26/52
Probability of selecting a 6 = 4/52
Probability of selecting both a black card and a 6 = 2/52
P(B or 6)= P(B) + P(6) – P(B and 6)
= 26/52 + 4/52 – 2/52
= 28/52
= 7/13.