MHB Expected value and variance of profit

Yankel
Messages
390
Reaction score
0
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
 
Physics news on Phys.org
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

$\chi$ $\sigma$
 
Hi all, I've been a roulette player for more than 10 years (although I took time off here and there) and it's only now that I'm trying to understand the physics of the game. Basically my strategy in roulette is to divide the wheel roughly into two halves (let's call them A and B). My theory is that in roulette there will invariably be variance. In other words, if A comes up 5 times in a row, B will be due to come up soon. However I have been proven wrong many times, and I have seen some...
Thread 'Detail of Diagonalization Lemma'
The following is more or less taken from page 6 of C. Smorynski's "Self-Reference and Modal Logic". (Springer, 1985) (I couldn't get raised brackets to indicate codification (Gödel numbering), so I use a box. The overline is assigning a name. The detail I would like clarification on is in the second step in the last line, where we have an m-overlined, and we substitute the expression for m. Are we saying that the name of a coded term is the same as the coded term? Thanks in advance.
Back
Top