You flip four fair coins

conditional probabilityprobability

You flip four fair coins, if you get 4 heads I will give you $10.00

You pay me $1.00 to play.

Should you play?

I want to have a discussion about the followup from this question, as I believe each play would be independent, but was told no it was conditional…

this means that you can keep playing the game, keeping the number of heads you already got and tossing the remaining coins. Every time you do this, you'll have to pay 1\$ to toss the remaining coins.

Best Answer

You shouldn't play since the probability of winning \$10 is $\frac {1} {2^4} = \frac{1}{16}$. So, the expected payout is $\frac{10}{16}<1$ (the cost of playing is 1\$).


EDIT: OP mentioned in the comments that you can actually continue playing this game. So, if you get 2 heads and 2 tails for example, you can pay another 1\$ and toss the two remaining coins and keep repeating this process till you get all 4 heads. This makes the problem slightly more interesting. We can now think of the process as a Markov chain with 5 states. Each state represents the number of heads accumulated so far (0 through 4). It is clear that you won't go backwards. If you have 2 heads for example, you can only go to states 2, 3 or 4 if you play again. The complete Markov matrix for this game becomes (some entries left blank for you to fill in) $$Q= \left[ \begin{matrix} \frac 1 {16} & . & . & . & \frac{1}{16}\\ 0 & \frac{1}{8} & . & . & \frac 1 8\\ 0 & 0 & \frac{1}{4} & \frac 1 2 & \frac 1 4\\ 0 & 0 & 0 & \frac 1 2 & \frac 1 2 \\ 0 & 0 & 0 & 0 & 1 \end{matrix} \right] $$

It is clear that the last state (4 heads) is an absorbing state. If you keep playing, how many times (on average) will you need to pay 1\$ before you reach the absorbing state of 4 heads and collect your 10\$ reward? This is the number of steps within transient states for the Markov matrix. Refer here for the fact that the average number of steps needed before getting to the 4\$ absorbing state is the very first entry of the vector:

$$t = (I-Q)^{-1} \Bbb I$$

Here, $I$ is the identity matrix and $\Bbb I$ is a vector of all ones. If this expected number of steps is less than 10, you should certainly play the game.


Another less precise and more intuitive way to look at this (if someone stops you on the street and offers you the opportunity and you can't invert matrices in your head) is that the number of remaining coins roughly halves on average every time you play. So, you should need on the order of $\log_2(4)$ tosses until all four coins become heads. This is roughly the amount you'll end up paying and much smaller than the 10\$ reward waiting for you.

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