Is there an optimal scenario?

A deck contains m + n m+n cards; m m cards are labeled 1 1 through m m in red \color{#D61F06}\text{red} , while the other n n are labeled 1 1 through n n in blue \color{#3D99F6}\text{blue} . Now, the deck is shuffled and placed face down. The rules of the game are as follows:

  • You draw cards from the pile one by one, and the moment a red \color{#D61F06}\text{red} number is drawn, you instantly lose.
  • So, if the first card you draw is a red \color{#D61F06}\text{red} number, regardless of its value, you lose the game right there.
  • However, if your first card is a blue \color{#3D99F6}\text{blue} number, it dictates how many cards you must draw (in total) to win the game. For example, if the first card you draw is 1 , {\color{#3D99F6}1}, you immediately win the game because you've already drawn a total of 1 card. If the first card is 3 , {\color{#3D99F6}3}, you must draw two more cards and the two have to be both blue for you to achieve victory.

To play this game, you must pay one dollar. But here is something interesting: before the game begins, you can decide how many red and blue cards to place in the deck, i.e. you can choose the values of m m and n . n. If you lose, you just lose $1. However, if you win, you receive your dollar back and for each red card included you receive one additional dollar.

What is the maximum expected return (in dollars) you can achieve with this game?


The answer is 0.

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1 solution

Plinio Sd
Jan 31, 2018

Let us find the probability p p of winning the game by deconditioning on the first picked card being the k k -th blue card. Then we have that p = ( i ) k = 1 n 1 n + m ( n 1 k 1 ) ( n + m 1 k 1 ) = k = 1 n 1 n ( n + m k m ) ( n + m m ) = ( i i ) 1 n ( n + m m ) i = 0 n 1 ( m + i m ) = ( i i i ) 1 n ( n + m m ) i = 0 n 1 [ ( m + i + 1 m + 1 ) ( m + i m + 1 ) ] = ( i v ) ( n + m m + 1 ) n ( n + m m ) = 1 m + 1 . \begin{aligned} p &\stackrel{(i)}{=} \sum_{k=1}^n \frac{1}{n+m}\frac{\binom{n-1}{k-1}}{\binom{n+m-1}{k-1}} \\ &= \sum_{k=1}^n \frac{1}{n}\frac{\binom{n+m-k}{m}}{\binom{n+m}{m}} \\ &\stackrel{(ii)}{=} \frac{1}{n\binom{n+m}{m}} \sum_{i= 0}^{n-1} \binom{m+i}{m} \\ &\stackrel{(iii)}{=} \frac{1}{n\binom{n+m}{m}} \sum_{i= 0}^{n-1} \left[ \binom{m+i+1}{m+1} - \binom{m+i}{m+1} \right] \\ &\stackrel{(iv)}{=} \frac{\binom{n+m}{m+1}}{n\binom{n+m}{m}} = \displaystyle\frac{1}{m+1}. \end{aligned} Step ( i ) (i) comes from the fact that the probability to take exactly the k k -th blue card is 1 / ( m + n ) 1/(m+n) and then, we need to take k 1 k-1 blue cards from the n 1 n-1 remaining. Step ( i i ) (ii) is the change of variable i = n k i=n-k . In ( i i i ) (iii) we used Pascal's rule . Finally, in step ( i v ) (iv) we have a telescoping series . Notice that p p does not depend on the number of blue cards! Finally, the expected return is simply E = p m + ( 1 p ) ( 1 ) = 0. E = p\,m + (1-p)\,(-1) = 0. Surprisingly, the expected return does not depend on n , m n,m . Therefore, the expected return is always zero!

What a great problem, Plinio... Thanks for posting!

Geoff Pilling - 3 years, 3 months ago

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Thank you for the kind comment!

Plinio SD - 3 years, 2 months ago

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