[Math] Expected sum of drawing 4 poker cards

probabilityprobability distributionsprobability theory

You have a deck of 52 cards. You draw 4, what is the expected value of the sum of those 4 cards that you draw?

Attempt:

Basically, the possible sum ranges from 4 to 52. Pr{sum = 4} = Pr{ 4 ace} = 4/52*3/51 *2/50*1/49

And then, you repeat this for sum =5, 6, 7, …. , 52. Then.

$E[Sum] =\sum_{i=4}^{53}i*Pr(i)$

I am wondering is there better/quicker approach besides manually finding each probability?

Best Answer

Elaborating on the comment by mlc, let $X_1,\ldots,X_4$ denote the four chosen cards, and let $\text{rank}(X_i)$ denote the rank (i.e., the face value, starting with $1$ for ace and ending with $13$ for king) of the card $X_i$ for $i=1,2,3,4$. The tuple $(X_1,X_2,X_3,X_4)$ is uniformly random from the set of all tuples of four distinct cards. Thus by symmetry, $X_1$ is uniformly random from the set of all $52$ cards (no card is favored more than the others), and therefore $$\mathbb E\ \text{rank}(X_1)=\frac{1+2+\cdots+13}{13}=7.$$ By the same argument, $\text{rank}(X_2),\text{rank}(X_3),$ and $\text{rank}(X_4)$ also have expected rank $7$.

Thus, by linearity of expectation, $$ \mathbb E(\text{rank}(X_1)+\text{rank}(X_2)+\text{rank}(X_3)+\text{rank}(X_4))=7+7+7+7=28. $$

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