From a standard 52-card deck of French playing cards, two random cards are missing(we don't know which ones). We pick 4 cards(without replacement). 'A' is such an event, where we pick exactly 3 aces. P(A)=?
We'll need to define a few events of the cards missing in order to approach this problem.
Let's define the three possible events of the cards missing first:
$0$ aces may be missing. Let's call this event $X$.
$1$ ace may be missing. Let's call this event $Y$.
$2$ aces may be missing. Let's call this event $Z$.
Note the importance of defining the above events in the context of our problem.
Now let's jump into the calculations:
Note the following:
$$P(A) = P(A \cap X) + P(A \cap Y) + P(A\cap Z)$$
Now use the fact that
$$P(A \cap B) = P(A|B)\times P(B)$$
Leaving out the details for you to work em out.