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In a new card game, you start with a well-shuffled full deck and draw 3 cards without replacement. If you draw 3 hearts, you win $$\$ 50$$. If you draw 3 black cards, you win $$\$ 25$$. For any other draws, you win nothing. (a) Create a probability model for the amount you win at this game, and find the expected winnings. Also compute the standard deviation of this distribution. (b) If the game costs $$\$ 5$$ to play, what would be the expected value and standard deviation of the net profit (or loss)? (Hint: profit \(=\) winnings \(-\) cost \(; X-5)\) (c) If the game costs \(\$ 5\) to play, should you play this game? Explain.

Short Answer

Expert verified
Don't play the game as expected profit is negative: \$-1.41.

Step by step solution

01

Calculate the probabilities of different outcomes

First, we need to determine the probability of each winning outcome. A deck has 52 cards, 13 of which are hearts and 26 of which are black (spades or clubs).1. **Probability of drawing 3 hearts:** There are 13 hearts, and we draw 3 without replacement. The number of ways to choose 3 hearts from 13 is given by the combination formula \( C(n, k) = \frac{n!}{k!(n-k)!} \), which results in \[ C(13, 3) = \frac{13 \times 12 \times 11}{3 \times 2 \times 1} = 286 \] The number of ways to choose any 3 cards from a deck of 52 is \[ C(52, 3) = \frac{52 \times 51 \times 50}{3 \times 2 \times 1} = 22100 \] So the probability of drawing 3 hearts is \[ P(3 \text{ hearts}) = \frac{286}{22100} \approx 0.01295 \]2. **Probability of drawing 3 black cards:** There are 26 black cards. Thus, the number of ways to choose 3 black cards is \[ C(26, 3) = \frac{26 \times 25 \times 24}{3 \times 2 \times 1} = 2600 \] So the probability of drawing 3 black cards is \[ P(3 \text{ black cards}) = \frac{2600}{22100} \approx 0.11765 \]3. **Probability of any other combination:** Since these are the only winning outcomes, the probability of a non-winning draw is: \[ P(\text{other}) = 1 - P(3 \text{ hearts}) - P(3 \text{ black cards}) \approx 1 - 0.01295 - 0.11765 = 0.8694 \]
02

Create the probability model for winnings

Now, create a table for the probability model based on the outcomes and winnings:- Win \\(50 for 3 hearts: Probability \( P(X=50) = 0.01295 \)- Win \\)25 for 3 black cards: Probability \( P(X=25) = 0.11765 \)- Win \$0 for any other: Probability \( P(X=0) = 0.8694 \)
03

Calculate expected winnings

The expected winnings can be calculated using the formula for expected value:\[ E[X] = \sum (x_i \cdot P(x_i)) \]- For \\(50: \(0.01295 \times 50\)- For \\)25: \(0.11765 \times 25\)- For \\(0: \(0.8694 \times 0\)Calculate:\[ E[X] = (0.01295 \times 50) + (0.11765 \times 25) + (0.8694 \times 0) \approx 0.6475 + 2.94125 + 0 = 3.58875 \]The expected winnings are approximately \\)3.59.
04

Calculate standard deviation of winnings

First, calculate the variance using the formula:\[ \text{Var}(X) = E[X^2] - (E[X])^2 \]Calculate \(E[X^2]\):- For \\(50: \(0.01295 \times 50^2 = 32.375\)- For \\)25: \(0.11765 \times 25^2 = 73.53125\)- For \$0: \(0.8694 \times 0^2 = 0\)\[ E[X^2] = 32.375 + 73.53125 + 0 = 105.90625 \]Now, find the variance:\[ \text{Var}(X) = 105.90625 - (3.58875)^2 = 105.90625 - 12.8788 \approx 93.02745 \]Finally, the standard deviation is:\[ \text{SD}(X) = \sqrt{93.02745} \approx 9.646 \]
05

Calculate expected net profit and standard deviation if it costs $5 to play

Subtract the cost of playing (\$5) from the expected winnings:- Expected net profit: \(E[X-5] = E[X] - 5 = 3.59 - 5 = -1.41\)The standard deviation remains the same as it is unaffected by constant shifts:- \(\text{SD}(X-5) = \text{SD}(X) = 9.646\)
06

Conclusion - Should you play the game?

Since the expected net profit is negative (\\(-1.41), you lose on average \\)1.41 per game. Considering the standard deviation, which suggests high variability, playing the game consistently results in a financial loss.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Expected Value
In probability theory, the expected value is like the weighted average of all possible outcomes of a random event, where each outcome is weighted by its probability of occurrence. Simply put, it's what you "expect" to get on average if you were to repeat the experiment many times. In mathematical terms, it’s calculated with the formula:
  • \( E[X] = \sum (x_i \cdot P(x_i)) \)
Here, \( x_i \) represents each possible outcome, and \( P(x_i) \) is the probability of each outcome.
In the context of the card game, to find the expected winnings, you multiply each reward by its respective probability and sum these products.
  • For winning \\(50: \( 50 \times 0.01295 \)
  • For winning \\)25: \( 25 \times 0.11765 \)
  • For winning \\(0: \( 0 \times 0.8694 \)
After calculating, the expected winnings amount to approximately \\)3.59. This figure tells you about the average amount you can expect to win with each game play over time.
However, when considering the cost of the game, this expected value decreases, revealing that you should anticipate a small loss on average after accounting for the playing fee.
Standard Deviation
Standard deviation is a measure used to quantify the amount of variation or dispersion of a set of values. In simpler terms, it tells you how much the outcomes of a probability distribution deviate from the expected value on average. The formula for standard deviation is:
  • \( \text{SD}(X) = \sqrt{\text{Var}(X)} \)
Where variance, \( \text{Var}(X) \), is given by:
  • \( E[X^2] - (E[X])^2 \)
To find the variance, calculate \( E[X^2] \), which is the expected value of the squares of the outcomes, and then subtract the square of the expected winnings. This step involves:
  • \( 0.01295 \times 50^2 \)
  • \( 0.11765 \times 25^2 \)
  • \( 0.8694 \times 0^2 \)
After finding \( E[X^2] = 105.90625 \) and subtracting \( (3.59)^2 \), you determine the variance to be approximately 93.03. The standard deviation then turns out to be around 9.646.
This high standard deviation indicates significant variability in potential winnings, meaning that while the average win might seem small, the actual outcome of each game could widely vary.
Combination Formula
The combination formula is a critical tool in probability that helps calculate the number of ways to choose a subset of items from a larger set. When order doesn’t matter, combinations are used. The formula is given by:
  • \( C(n, k) = \frac{n!}{k!(n-k)!} \)
where \( n \) is the total number of items, \( k \) is the number of items to choose, and \(!\) denotes factorial, which is the product of all positive integers up to that number.
In the card game, the formula is used to find the probability of certain hands being drawn. For instance:
  • Choosing 3 hearts from 13 available hearts: \( C(13, 3) \)
  • Choosing 3 cards from the full deck: \( C(52, 3) \)
The use of this formula simplifies the calculation of probabilities, where, for example, the probability of drawing three hearts was found by dividing \( C(13, 3) \) by \( C(52, 3) \).
Understanding and applying the combination formula is crucial for calculating probabilities in games involving card draws, lotteries, and other similar scenarios where order is irrelevant.

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