MHB Expected value and variance of profit

AI Thread Summary
The discussion revolves around calculating the expected value and variance of profit for a man playing a game against a machine. The machine randomly selects two numbers from a set, and the man earns $5 if their product is even, after paying $2 to play each game. The probability of an even product is calculated as 16/25, leading to a profit of either $3 or -$2 per game. The expected profit after 100 games is determined to be $160, while the variance is calculated as 108.8, indicating that the overall expected profit is negative when considering the initial cost to play. Thus, the game is not financially favorable for the player.
Yankel
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Hello all, I have this question, which I think I partially knows how to solve, but need some completion.

"A man is playing versus a machine in the following way: The machine chooses 2 numbers randomly from the set of numbers 1,2,3,4,5, where a number can be chosen twice (with replacement). If the multiplication of the 2 chosen numbers is even, the man gets 5 dollars Calculate the expected value (mean) and variance of the profit after 100 games, if for every game he pays 2 dollars to play."

What I did to start with, is to calculate the probability of having an even multiplication and I got p=16/25. Now I know I can calculate the profit for a single game, get a probability function with 2 values, and find E(X), and multiply it by 100. To be more specific, if X is the profit in 1 game, then it can get the values 3 and -2 only. The matching probabilities are 16/25 and 9/25 respectively. Therefore it's easy to find E(X) and V(X). However I need the expected value and variance after 100 games. I know that for E it's 100*E(X), what about V ?

Thank you
 
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Your calculations are correct in my opinion if you assume that the order of picking two random numbers in the set $\{1,2,3,4,5\}$ does matter.

If you denote $\{X_1,\ldots,X_{100}\}$ as the sequence of profits of the first $100$ games then I think you can assume this random variables are independent and Bernoulli distributed. Hence their sum $S_{100} = \sum_{j=1}^{100} X_j$ represents the pay-off after the $100$th game which has a Binomial distribution.
 
Yankel said:
Hello all, I have this question, which I think I partially knows how to solve, but need some completion.

"A man is playing versus a machine in the following way: The machine chooses 2 numbers randomly from the set of numbers 1,2,3,4,5, where a number can be chosen twice (with replacement). If the multiplication of the 2 chosen numbers is even, the man gets 5 dollars Calculate the expected value (mean) and variance of the profit after 100 games, if for every game he pays 2 dollars to play."

What I did to start with, is to calculate the probability of having an even multiplication and I got p=16/25. Now I know I can calculate the profit for a single game, get a probability function with 2 values, and find E(X), and multiply it by 100. To be more specific, if X is the profit in 1 game, then it can get the values 3 and -2 only. The matching probabilities are 16/25 and 9/25 respectively. Therefore it's easy to find E(X) and V(X). However I need the expected value and variance after 100 games. I know that for E it's 100*E(X), what about V ?

Thank you

At each game the probability of winning is...

$\displaystyle p = \frac{1}{5}\ \frac{2}{5} + \frac{1}{5} + \frac{1}{5}\ \frac{2}{5} + \frac{1}{5} + \frac{1}{5}\ \frac{2}{5} = \frac{8}{25}\ (1)$

For a binomial distribution the expected number of successes in n games is...

$\displaystyle \mu = n\ p\ (2)$

... and the variance...

$\displaystyle \sigma= n\ p\ (1-p)\ (3)$

In your case is $n=100$ and $p = \frac{8}{25}$, so that the expected profit is $E = 32\ 5 = 160$ with variance $V= 32\ \frac{17}{25}\ 5 = \frac{1088}{5} = 108.8$ ... taking into account that if You want to play 100 games You have to pay 200 dollars, the global expected profit is negative...

Kind regards

$\chi$ $\sigma$
 
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