[Math] Probability of 3 Events – Standard Card Deck

probability

I am trying to solve the following question.

Suppose you select a card at random from a standard deck, and then without putting it back, you select a second card at random from the remaining 51 cards. What is the probability that both cards have the same rank, or both have the same suit, or one is red and one is black (diamonds and hearts-red, clubs and spades-black?

For this problem, can I use the following formula for the union of these three events?

$$P(E\cup F\cup G)=P(E)+P(F)+P(G)-P(E\cap F)-P(E\cap G)-P(F\cap G)+P(E\cap F\cap G)$$

With probability that both cards have same rank being 13/52 * 12/52, probability that both have same suit being 26/52 * 25/52, and probability that one is red and one is black being 1/2 * 1/2? Am I on the right track?

Best Answer

Looks to me like you're overcomplicating this. Answering the following question should help you

  • Does the identity of the first card you pick change the number of cards which you can pick second that will result in a match (i.e. your criteria being met)?

The answer to this question is no. It doesn't matter, because every card has the same properties relative to the others (3 cards with same value, 12 with same suit, 25 with same colour, etc).

So assume your card is the Ace of Spades, for argument's sake.

Then a "winning" card is any of the following:

  1. Any red card (26)
  2. The Ace of Clubs (1) [note Aces of Hearts and Diamonds are red cards]
  3. Any other spade (12)

So there are $39$ winning cards out of $51$, giving a probability of $\frac{39}{51}$