2023 AIME I Problems/Problem 6

Problem

Alice knows that red cards and black cards will be revealed to her one at a time in random order. Before each card is revealed, Alice must guess its color. If Alice plays optimally, the expected number of cards she will guess correctly is where and are relatively prime positive integers. Find

Solution 1 (Casework)

We break the problem into stages, one for each card revealed, then further into cases based on the number of remaining unrevealed cards of each color. Since expected value is linear, the expected value of the total number of correct card color guesses across all stages is the sum of the expected values of the number of correct card color guesses at each stage; that is, we add the probabilities of correctly guessing the color at each stage to get the final answer (See https://brilliant.org/wiki/linearity-of-expectation/)

At any stage, if there are unrevealed cards of one color and of the other color, and , then the optimal strategy is to guess the color with unrevealed cards, which succeeds with probability

Stage 1:

There are always unrevealed cards of each color, so the probability of guessing correctly is .

Stage 2:

There is always a - split ( unrevealed cards of one color and of the other color), so the probability of guessing correctly is .

Stage 3:

There are now cases:

• The guess from Stage 2 was correct, so there is now a - split of cards and a probability of guessing the color of the third card correctly.
• The guess from Stage 2 was incorrect, so the split is - and the probability of guessing correctly is .

Thus, the overall probability of guessing correctly is .

Stage 4:

This stage has cases as well:

• The guesses from both Stage 2 and Stage 3 were incorrect. This occurs with probability and results in a - split and a certain correct guess at this stage.
• Otherwise, there must be a - split and a probability of guessing correctly.

The probability of guessing the fourth card correctly is therefore .

Stage 5:

Yet again, there are cases:

• In Stage 4, there was a - split and the guess was correct. This occurs with probability and results in a - split with a chance of a correct guess here.
• Otherwise, there must be a - split, making a correct guess certain.

In total, the fifth card can be guessed correctly with probability .

Stage 6:

At this point, only card remains, so the probability of guessing its color correctly is .

In conclusion, the expected value of the number of cards guessed correctly is so the answer is

~OrangeQuail9

Solution 2 (Casework)

At any point in the game, Alice should guess whichever color has come up less frequently thus far (although if both colors have come up equally often, she may guess whichever she likes); using this strategy, her probability of guessing correctly is at least on any given card, as desired.

There are possible orderings of cards, all equally likely (since any of the permutations of the cards is equally likely, and each ordering covers permutations).

Each of the orderings that start with red cards corresponds with one that starts with a black card; the problem is symmetrical with respect to red and black cards, so we can, without loss of generality, consider only the orderings that start with red cards.

We then generate a tally table showing whether Alice's guesses are correct for each ordering; for a given card, she guesses correctly if fewer than half the previously shown cards were the same color, guesses incorrectly if more than half were the same color, and guesses correctly with probability if exactly half were the same color.

In this table, denotes a correct guess, denotes an incorrect guess, and denotes a guess with probability of being correct.

Now we sum the tallies across orderings, obtaining , and finally divide by the number of orderings () to obtain the expected number of correct guesses, , which yields an answer of

~IndigoEagle108

Solution 3 (Dynamic Programming)

Denote by the optimal expected number of cards that Alice guesses correctly, where the number of red and black cards are and , respectively.

Thus, for , we have

For , Alice always guesses black. So .

For , Alice always guesses red. So .

To solve this dynamic program, we can also exploit its symmetry that .

By solving this dynamic program, we get , , , , , .

Therefore, the answer is .

~Steven Chen (Professor Chen Education Palace, www.professorchenedu.com)

See also

The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.