Probability – Drawing an Ace Card from a Deck of 52 on the nth Try

probability

I have the following question:

We draw one card at a time without replacement from the top of a shuffled standard poker deck and stop when we draw an ace. Let $X$ be the number of cards we have drawn, then calculate the probabilities for $X=10$, $X=50$, and $X < 10$.

I have a theory as to how to solve the first one: calculate the probability of the drawing $10$ cards WITHOUT getting an ace, which has a probability of $\binom{48}{10} / \binom{52}{10}$, and subtracting this from $1$. For the other two, I am a little bit lost.

Regarding the second one, I think that if I draw $50$ cards from the deck, the probability of drawing an ace should be $1$, shouldn't it?

And unfortunately I'm lost on the last part. I would love a clue on this matter.

Thanks

Best Answer

$P(X = k)$ is the probability that the first ace appears in the $k$th position.

What is $P(X = 10)$?

For the first ace to appear in the tenth position, we must first choose nine non-aces, then choose an ace with the tenth draw. The probability that the first nine cards are not aces is $$\frac{\dbinom{48}{9}}{\dbinom{52}{9}}$$ The probability that one of the four aces is chosen as the tenth card from the $43$ remaining cards is $$\frac{4}{43}$$ Hence, the desired probability is $$\frac{\dbinom{48}{9}}{\dbinom{52}{9}} \cdot \frac{4}{43}$$

What is $P(X = 50)$?

Since there are four aces in the $52$-card deck, the first ace must appear no later than the $49$th position. Hence, $P(X = 50) = 0$.

What is $P(X < 10)$?

The probability that $P(X < 10)$ is found by subtracting the probability that the first nine cards are not aces from $1$.

$$P(X < 10) = 1 - \frac{\dbinom{48}{9}}{\dbinom{52}{9}}$$

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