So, lets illustrate that with an example.

We have a deck of 52 cards, contains three jacks, three queens, and three kings.

What is the probability of drawing either a king or a queen from a full deck?

So which of these alternatives is it?

So in this case we use this equation for

this probability that we just had.

So in this case, the probability of A occurring,

which is the probability of drawing a king.

There are 3 kings and 52 cards, so the probability of drawing a king is 3/52,

first one here.

Similarly, the probability of drawing a queen P(B) is

also equal to 3/52 because there are three queens out of 52.

And finally the joint probability is zero,

because can't draw two cards at the same time here.

So therefore, the probability is just the sum of the individual probabilities,

6/52 or 0.115, so the answer is B.

Next example, five people gather for a dinner party.

The probability that no two of them have the same birthday is

which of these alternatives?