# Probability of deck of cards such that each person receives one ace

Suppose that a deck of 52 cards containing four aces is shuffled thoroughly and the cards are then distributed among four players so that each player receives 13 cards. Determine the probability that each player will receive one ace.

The answer to this is given as$$\frac{13^4}{\binom {52}4}$$

My doubt is the following:

1. The book justifies ${\binom {52}{4}}$ as number of possible different combinations of the four positions in the deck occupied by 4 aces. That sounds like a case of arrangements to me, so shouldn't we think about permutations and not combinations if we are concerned about how the aces are to be arranged in the deck ?.
2. Shouldn't the denominator be ${\binom {52}{13}}$ since you are choosing 13 cards for 4 people.
• We don't care who gets which ace, nor which hand each person is getting. The blocks of thirteen, and the aces, are two sets within which every member is equivalent. – Nij Nov 15 '17 at 5:31
• I don't completely understand. Could you please elaborate. Shouldn't the sample space consist of all combinations of 13 draws in which aces might or might not be there – madhavU Nov 15 '17 at 5:35
• Guess I get it now. So its like saying that we have 52 positions out of which we need to choose 4 positions for the aces. 52C13 would imply we take into consideration the rest of the cards as well which is not needed in this case. – madhavU Nov 15 '17 at 6:06
• It completely depends on who shuffles the deck and how it is shuffled. Should we assume some specific distribution? – mathreadler Nov 15 '17 at 8:55
• I don't think the question asks us to assume a specific distribution mathreadler – madhavU Nov 15 '17 at 14:41

There are $$\binom{52}{13}\binom{39}{13}\binom{26}{13}\binom{13}{13}$$ ways to distribute $13$ cards to each of four people.

There are $4!$ ways to distribute the aces so that each person receives one and $$\binom{48}{12}\binom{36}{12}\binom{24}{12}\binom{12}{12}$$ ways to distribute the remaining cards so that each person receives twelve of them. Hence, the desired probability is \begin{align*} \frac{4!\dbinom{48}{12}\dbinom{36}{12}\dbinom{24}{12}\dbinom{12}{12}}{\dbinom{52}{13}\dbinom{39}{13}\dbinom{26}{13}\dbinom{13}{13}} & = \frac{4! \cdot \dfrac{48!}{12!36!} \cdot \dfrac{36!}{12!24!} \cdot \dfrac{24!}{12!12!} \cdot \dfrac{12!}{12!0!}}{\dfrac{52!}{13!39!} \cdot \dfrac{39!}{13!26!} \cdot \dfrac{26!}{13!13!} \cdot \dfrac{13!}{13!0!}}\\[2mm] & = \frac{4! \cdot \dfrac{48!}{12!12!12!12!}}{\dfrac{52!}{13!13!13!13!}}\\[2mm] & = \frac{4!48!}{12!12!12!12!} \cdot \frac{13!13!13!13!}{52!}\\[2mm] & = \frac{4!48!13^4}{52!}\\[2mm] & = \frac{13^4}{\dfrac{52!}{4!48!}}\\[2mm] & = \frac{13^4}{\dbinom{52}{4}} \end{align*}

Let's compare this solution with the approach of your author. As you stated, there are $\binom{52}{4}$ ways to choose the four positions occupied by the aces in the deck. Since each person receives $13$ cards, there are $13$ possible places for the position of the ace in each person's hand. Hence, the desired probability is $$\frac{13^4}{\dbinom{52}{4}}$$

• That's a wonderful answer. Thanks a lot N.F Taussig. I have a silly doubt, if your sample space does not include arrangements, then why have you included the possibility of arrangements in the numerator ?. – madhavU Nov 15 '17 at 14:40
• If you are referring to the text's answer, notice that we are choosing positions for the aces in both the numerator and the denominator. In the numerator, we are choosing the position for the ace in each person's hand. In the denominator, we are choosing the position of the aces in the deck. – N. F. Taussig Nov 15 '17 at 19:49

All we really care about is the placement for the aces; not where the other cards may be in the deck, nor even the suits of the aces.

So let us take 52 blank cards and 4 stickers with 'ace' written on them.   To 'deal the cards', place the blank cards in for lines of 13, then unbiasedly select four from them and put a sticker on each.

How may ways are there to stick the aces on 4 different cards in the deck of 52? That is a selection of 4 from 52. $$\binom {52}4$$

How many ways are there to do this so that each ace is stuck on one card in each from the four lines of 13? That is a selection of 1 from the first line, 1 from the next line, and so forth. $$\binom{13}1\binom{13}1\binom{13}1\binom{13}1$$

Divide and calculate.$$\dfrac{13^4}{\dbinom{52}{4}}$$

Informal analysis: we can re-imagine the process of the deal as one in which there are 52 slots, 13 assigned to each hand and the cards are distributed from the top unifomly to the remaining slots. We put the four aces on the top of the deck. The first ace can be dealt to any of the four players. The second ace can be dealt to any of the remaining 39 slots. The third ace can be dealt to any of the remaining 26 slots. The last ace can be dealt to 13 slots. So odds of this happening are: $$\begin{equation*} {52 \over 52} \times {39 \over 51} \times {26 \over 50} \times {13 \over 49} \approx 10.5\% \end{equation*}$$ Formal approach: first, we have to identify our probability space. Using the multiinomial coefficient we can count the total number of atomic events as $$|\Omega| = {52 \choose 13,13,13,13}$$. We consider how remaining cards are dealts, 12 to each of the remaining players, and each player has one ace. So $$|N_{\text{one ace each}}| = 4! \cdot {48 \choose 12, 12, 12, 12}$$. So we can compute: \begin{align} \Pr(N_{\text{one ace each}}) &= \frac{|N_{\text{one ace each}}|}{|\Omega|} \\ &= {4! \cdot {48 \choose 12,12,12,12} \over {52 \choose 13,13,13,13}}\\ &= {{52 \times 39 \times 26 \times 13} \over {52 \times 51 \times 50 \times 49}}\\ &\approx 10.5\% \end{align}