[Math] the probability of being dealt a hand where all $5$ cards are different denominations

combinatoricsprobability

In a game of poker you are dealt $5$ cards at random from a standard deck of $52$. A standard deck of $52$ contains $13$ different denominations of cards (Ace, 2, 3, …, 10, jack, queen, king), each in four different suits

(a) What is the probability of being dealt a hand where all $5$ cards are different denominations?

(b) What is the probability of being dealt a hand that includes at least one Ace?

My attempts:

(a) Let D = the event where a hand is dealt such that all $5$ cards are different denominations.

There are ($\binom{13}{5}$ ways to pick the denominations $\times \binom{4}{1}$ ways to pick the suit)/ ($\binom{52}{5}$ total combinations)

(b) Let A = the event where a hand is dealt such that it includes at least one ace.

$P(a) =$ ($\binom{4}{1}$ ways to pick the ace $\times \binom{13}{4}$ ways to pick the other cards $\times \binom{4}{1}$ ways to pick the suit)/ ($\binom{52}{5}$ total combinations)

If someone could walk me through a problem similar to this it'd be really appreciated this stuff is sort of confusing. (poor cs major)

Best Answer

(a) What is the probability of being dealt a hand where all 5 cards are different denominations?

We can multiply together the probabilities that have each successive draw satisfy the condition. $$\frac{52}{52}\cdot \frac{48}{51}\cdot\frac{44}{50}\cdot\frac{40}{49}\cdot\frac{36}{48}=\frac{2112}{4165}\approx0.50708$$ On the first draw, you have a probability of $\frac{52}{52}$ of selecting a card that is different from a card in your hand (naturally, as you have no cards in your hand).
On the second draw, you have a probability of $\frac{48}{51}$ of selecting a card that is different from the card you had just drawn (that is, only $\frac3{51}$ match the card in your hand).
Apply for the next three draws.

(b) What is the probability of being dealt a hand that includes at least one Ace?

To get at least one Ace, we must fail the action of getting no Aces. $$1-\frac{48}{52}\cdot\frac{47}{51}\cdot\frac{46}{50}\cdot\frac{45}{49}\cdot\frac{44}{48}=1-\frac{35673}{54145}\approx0.34116$$ Alternatively, we can think of this as the number of ways to choose $5$ from $48$ (non-Aces), out of the number of ways to choose $5$ from $52$ (total cards in deck): $$1-\frac{\binom{48}5}{\binom{52}5}=1-\frac{\frac{48!}{5!\ (48-5)!}}{\frac{52!}{5!\ (52-5)!}}=1-\frac{48!\cdot47!}{43!\cdot52!}=1-\frac{48}{52}\cdot\frac{47}{51}\cdot\frac{46}{50}\cdot\frac{45}{49}\cdot\frac{44}{48}=1-\frac{35673}{54145}\approx0.34116$$ The same calculation! Seems the suits did not have anything to do with the probabilities, after all!