A deck contains cards; cards are labeled through in , while the other are labeled through in . Now, the deck is shuffled and placed face down. The rules of the game are as follows:
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 and 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?
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Let us find the probability p of winning the game by deconditioning on the first picked card being the k -th blue card. Then we have that p = ( i ) k = 1 ∑ n n + m 1 ( k − 1 n + m − 1 ) ( k − 1 n − 1 ) = k = 1 ∑ n n 1 ( m n + m ) ( m n + m − k ) = ( i i ) n ( m n + m ) 1 i = 0 ∑ n − 1 ( m m + i ) = ( i i i ) n ( m n + m ) 1 i = 0 ∑ n − 1 [ ( m + 1 m + i + 1 ) − ( m + 1 m + i ) ] = ( i v ) n ( m n + m ) ( m + 1 n + m ) = m + 1 1 . Step ( i ) comes from the fact that the probability to take exactly the k -th blue card is 1 / ( m + n ) and then, we need to take k − 1 blue cards from the n − 1 remaining. Step ( i i ) is the change of variable i = n − k . In ( i i i ) we used Pascal's rule . Finally, in step ( i v ) we have a telescoping series . Notice that p does not depend on the number of blue cards! Finally, the expected return is simply E = p m + ( 1 − p ) ( − 1 ) = 0 . Surprisingly, the expected return does not depend on n , m . Therefore, the expected return is always zero!