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I encountered the following problem:

From a shuffled deck the cards are flipped one by one until a queen shows up. Then again a card is flipped. Is it more likely that this card is the Jack of Spades or the Queen of Hearts?

I managed to solve this by first finding the probability that the card will be the Jack of Spades under the extra condition that the rank of the Jack of Spades is $n$. If $J_s$ denotes the event that the Jack of Spades is the card and $N$ denotes the rank of the Jack of Spades then:$$P(J_s\mid N=n)=\binom{52-n}3\times\binom{51}4^{-1}$$This because under condition $N=n$ there are $\binom{51}4$ possibilities for the $4$ queens of which $\binom{52-n}3$ are favorable.

Applying the hockey stick identity and the law of total probability we then find: $$P(J_s)=\frac1{52}$$From this it can be deduced easily that also $P(Q_h)=\frac1{52}$ so apparantly the chances are equal. This amazed me because I could not find an explaining symmetry for that.

My question arises from the fact that I am simply not satisfied with this solution and is:

Could someone give me a "nicer" solution that avoids calculations and rests on something like a smart perspective of the case?

I just feel that a nicer and more direct solution can be found.

Thank you for taking notice of my question, which is purely an effort to enrich my intuitions on probability.


Edit

Thank you for comments already, but if you have a real answer then please do not hesitate to provide one. And preferably a complete one. Also: more perspectives will imply a larger enrichment of my intuition.

Thank you in advance.

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  • $\begingroup$ "From a shuffled deck the cards are flipped one by one until a queen shows up". At this step, you can consider 4 options. If this queen is Club, Diamond, Hearts, Spade... for each option, try to continue. $\endgroup$
    – Lourrran
    Commented Mar 8, 2023 at 10:34
  • $\begingroup$ Just to be clear, I suppose that by "rank" you mean position in the queue; and that there is a presumption that when the first Queen turns up, the Queen of Hearts and Jack of Spades are still to come ? $\endgroup$ Commented Mar 8, 2023 at 10:58
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    $\begingroup$ For every card $X$ different than the first queen, the probability of "$X$ follows the first queen" is the same. However, for the first queen itself, the probability it follows itself is zero. Of course the jack of spades can not be the first queen – but the queen of hearts can. $\endgroup$
    – CiaPan
    Commented Mar 8, 2023 at 10:59
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    $\begingroup$ I don't think that the probabilities are the same because it could be the case that the first queen will be the queen of hearts. $\endgroup$
    – Masacroso
    Commented Mar 8, 2023 at 11:08
  • $\begingroup$ @trueblueanil By "rank" I mean indeed the position in the queue. But it is not excluded that the Jack of Spades has a smaller rank than the first Queen. Also it is not excluded that the first Queen is the Queen of hearts. If not then it is for sure the Queen of hearts has a larger rank than the first Queen. $\endgroup$
    – drhab
    Commented Mar 8, 2023 at 11:10

3 Answers 3

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Take the card you are interested in, be it $Q\heartsuit$, $J\spadesuit$, or something else entirely, call it $X$. Now remove $X$ from the deck and sort the rest of the cards. There are, of course, $51!$ ways to sort the $X-$less deck, and then there is a unique spot in which to insert the $X$ so that it immediately follows the first queen.

In this way, we see that there are exactly $51!$ arrangements of the deck such that $X$ is immediately following the first Queen. As there are $52!$ ways to sort the deck without worrying about $X$, the probability that $X$ is in the desired slot is $$\frac {51!}{52!}=\frac 1{52}$$

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    $\begingroup$ Ah, yes. It has landed (and you are a genius)! Thank you very much, lulu. Especially for confirmation of my conjecture and enrichment of my intuition. $\endgroup$
    – drhab
    Commented Mar 8, 2023 at 12:05
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    $\begingroup$ @drhab It does tend to resist intuition. The notion is something like "sure, the specified Queen might have already been drawn as the first Queen but then you probably drew a whole lot of non-queen cards en route to getting that first queen and these two effects cancel." Hard to make that precise though. Good problem. $\endgroup$
    – lulu
    Commented Mar 8, 2023 at 13:22
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    $\begingroup$ @trueblueanil Of course it is. Just as it is "game over" if you draw $J\spadesuit$ in the run prior to getting the first Queen. $\endgroup$
    – lulu
    Commented Mar 8, 2023 at 15:15
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    $\begingroup$ @trueblueanil Try it with three cards, $J, Q_1, Q_2$. Easy to see, by direct enumeration, that each of the three cards is equally likely to be chosen immediately after the first $Q$. $\endgroup$
    – lulu
    Commented Mar 8, 2023 at 15:17
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    $\begingroup$ @trueblueanil Correct me if I am wrong, but I think your intuition is based on the following false argument: if you condition on the first $Q$ being $Q\heartsuit$ then it is impossible for the next card to also be $Q\heartsuit$ but there is a $\frac 1{51}$ chance that it is $J\spadesuit$. If you condition on the first $Q$ being something other than a heart, then both have a $\frac 1{51}$ chance of being next. But this is wrong because in the second path, we are sure that no queen was chosen in the initial run but it is perfectly possible that $J\spadesuit$ was, so there is no equality. $\endgroup$
    – lulu
    Commented Mar 8, 2023 at 15:23
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Here’s another perspective, by far not as elegant as the one in lulu’s answer, but perhaps it throws some more light on what misleads our intuition on this nice problem.

A quantitative form of the argument for the wrong intuition that the jack should be more likely would be: The probability that $Q\heartsuit$ has already been drawn after the first queen is drawn is $\frac14$, whereas the probability that $J\spadesuit$ has already been drawn is $\frac15$ (since that card and the four queens are all equally likely to be drawn first). Thus it’s more likely that $J\spadesuit$ is still in the deck than that $Q\heartsuit$ is still in the deck.

The mistake here is that these are just the marginal probabilities, but these events correlate with how many cards are still in the deck, and while $J\spadesuit$ is more likely to still be in the deck, in that case the deck is more likely to still contain more cards, which makes it less likely that $J\spadesuit$ will be drawn next.

The probability that, after the first queen is drawn, $Q\heartsuit$ has been drawn and there are $k$ cards left is

$$ \mathsf{Pr}(Q\heartsuit\text{ drawn}\land K=k)=\frac14\frac{\binom k3}{\binom{52}4}\;, $$

since there are $\binom{52}4$ ways to choose where the $4$ queens are, and for the event to occur $Q\heartsuit$ has to be in a particular place and the remaining $3$ queens must be among the remaining $k$ cards.

The probability that, after the first queen is drawn, $J\spadesuit$ has been drawn and there are $k$ cards left is

$$ \mathsf{Pr}(J\spadesuit\text{ drawn}\land K=k)=\frac{51-k}{48}\frac{\binom k3}{\binom{52}4}\;, $$

since one queen must be in a particular place, the other three must be among the remaining $k$ cards and $J\spadesuit$ must be among the $51-k$ cards drawn before the first queen, and there are $48$ places left where it could be.

Summing these over $k$ yields $\frac14$ and $\frac15$, respectively, as it must. But to get the probability that $Q\heartsuit$ or $J\spadesuit$ is drawn next, we need to sum the probability that that card hasn’t been drawn times $\frac1k$:

\begin{eqnarray} \mathsf{Pr}(Q\heartsuit\text{ is next}) &=& \sum_{k=3}^{51}\mathsf{Pr}(Q\heartsuit\text{ not drawn}\land K=k)\cdot\frac1k \\ &=& \sum_{k=3}^{51}\left(1-\frac14\right)\frac{\binom k3}{\binom{52}4}\cdot\frac1k \\ &=& \frac1{52} \end{eqnarray}

and

\begin{eqnarray} \mathsf{Pr}(J\spadesuit\text{ is next}) &=& \sum_{k=3}^{51}\mathsf{Pr}(J\spadesuit\text{ not drawn}\land K=k)\cdot\frac1k \\ &=& \sum_{k=3}^{51}\left(1-\frac{51-k}{48}\right)\frac{\binom k3}{\binom{52}4}\cdot\frac1k \\ &=& \frac1{52}\;. \end{eqnarray}

Here’s a Wolfram|Alpha plot of $\mathsf{Pr}(Q\heartsuit\text{ not drawn}\land K=k)$ and $\mathsf{Pr}(J\spadesuit\text{ not drawn}\land K=k)$ over $k$. You can see that the area under the curve for $J\spadesuit$ is a bit larger, but more of that area is at high $k$, whereas the probability for $Q\heartsuit$ is higher than the one for $J\spadesuit$ for $k\lt39$. The expected value of $k$ given that $Q\heartsuit$ or $J\spadesuit$ is still in the deck is

\begin{eqnarray} \mathsf E[K\mid Q\heartsuit\text{ not drawn}] &=& \frac43\sum_{k=3}^{51}\mathsf{Pr}(Q\heartsuit\text{ not drawn}\land K=k)\cdot k \\ &=& \frac43\sum_{k=3}^{51}\left(1-\frac14\right)\frac{\binom k3}{\binom{52}4}\cdot k \\ &=& \frac{207}5 \\ &=& 41.4 \end{eqnarray}

and

\begin{eqnarray} \mathsf E[K\mid J\spadesuit\text{ not drawn}] &=& \frac54\sum_{k=3}^{51}\mathsf{Pr}(J\spadesuit\text{ not drawn}\land K=k)\cdot k \\ &=& \frac54\sum_{k=3}^{51}\left(1-\frac{51-k}{48}\right)\frac{\binom k3}{\binom{52}4}\cdot k \\ &=&\frac{259}6 \\ &=&43.1\overline6\;, \end{eqnarray}

respectively, not that much of a difference, but what matters is the probabilities for small values of $k$, where it’s most likely to actually draw the card next if it’s still there, and since the ratio of the two probabilities is $\frac{k-3}{36}$, the queen has a considerable advantage at small $k$. This, of course, goes back to what’s already been discussed in comments, that $J\spadesuit$ can be gone due to having been drawn as any of the cards before the first queen, whereas $Q\heartsuit$ can only be gone due to being the first queen, which always has the same probability of $\frac14$ no matter how many cards have already been drawn.

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  • $\begingroup$ Thank you, Joriki. This answer indeed pictures nicely what is actually going on and pinpoints where our intuitions can easily fool us. $\endgroup$
    – drhab
    Commented Mar 9, 2023 at 9:33
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Edited: We know absolutely nothing about the arrangement. Therefore each card is as likely to be any card, including the card after the first queen.

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  • $\begingroup$ The picture I have in mind is: we place all cards in a row one by one and place JoS as last card. After placing the first card there are $2$ possibilities for placing the second (on left or on right of the first card). Then after placing the second there are $3$ possibilities for the third (left, middle, right). Et cetera. If we place the JoS as last then there are $52$ possible spots and exactly one is favorable in the sense that it is exactly on the right of the first queen. Same story if we place QoH as last card. Does that agree with what you are trying to say here? $\endgroup$
    – drhab
    Commented Mar 10, 2023 at 14:00
  • $\begingroup$ @drhab My idea is a little different, but it boils down to the same I guess. My idea is that there are 52 slots for the cards, and each card is in one slot. The JoS could be in any one of the 52 slots with the same probability, but it is only in one. If that slot is the slot after the first ace, then that's the favorable case. The probability that the card is in that slot is 1/52. Same for the QoH. $\endgroup$
    – Ed Jansen
    Commented Mar 10, 2023 at 17:06
  • $\begingroup$ @drhab correction: my above comment should say 'after the first queen' instead of 'after the first ace'. $\endgroup$
    – Ed Jansen
    Commented Mar 10, 2023 at 20:22

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