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Pair-a-dice Suppose you roll a pair of fair, six-sided dice. Let \(T=\) the sum of the spots showing on the up-faces. (a) Find the probability distribution of \(T\) (b) Make a histogram of the probability distribution. Describe what you see. (c) Find \(P(T \geq 5)\) and interpret the result.

Short Answer

Expert verified
The probability distribution is symmetric around T=7. \(P(T \geq 5) = \frac{13}{18}\).

Step by step solution

01

Understand the Possible Outcomes

When you roll two six-sided dice, each die has 6 faces, so there are 6 \( \times \) 6 = 36 equally likely outcomes. The sum of the spots can range from 2 (1+1) to 12 (6+6).
02

Construct the Probability Distribution

To find the probability of each sum \( T \), count how many ways each sum can occur.\( \begin{aligned}&\text{Sum 2: 1 way (1,1)} \ &\text{Sum 3: 2 ways (1,2),(2,1)} \ &\text{Sum 4: 3 ways (1,3),(2,2),(3,1)} \ &\text{Sum 5: 4 ways (1,4),(2,3),(3,2),(4,1)} \ &\text{Sum 6: 5 ways (1,5),(2,4),(3,3),(4,2),(5,1)} \ &\text{Sum 7: 6 ways (1,6),(2,5),(3,4),(4,3),(5,2),(6,1)} \ &\text{Sum 8: 5 ways (2,6),(3,5),(4,4),(5,3),(6,2)} \ &\text{Sum 9: 4 ways (3,6),(4,5),(5,4),(6,3)} \ &\text{Sum 10: 3 ways (4,6),(5,5),(6,4)} \ &\text{Sum 11: 2 ways (5,6),(6,5)} \ &\text{Sum 12: 1 way (6,6)}\end{aligned} \)So the probability distribution is explained as follows:\[ P(T = x) = \frac{\text{Number of ways to get sum } x}{36} \]
03

Calculate Specific Probabilities

Relating the earlier counts, we can conclude:\(\begin{aligned}&P(T=2) = \frac{1}{36} \&P(T=3) = \frac{2}{36} = \frac{1}{18} \&P(T=4) = \frac{3}{36} = \frac{1}{12} \&P(T=5) = \frac{4}{36} = \frac{1}{9} \&P(T=6) = \frac{5}{36} \&P(T=7) = \frac{6}{36} = \frac{1}{6} \&P(T=8) = \frac{5}{36} \&P(T=9) = \frac{4}{36} = \frac{1}{9} \&P(T=10) = \frac{3}{36} = \frac{1}{12} \&P(T=11) = \frac{2}{36} = \frac{1}{18} \&P(T=12) = \frac{1}{36}\end{aligned}\)
04

Draw the Histogram

A histogram will show the sums on the x-axis from 2 to 12 and the probabilities on the y-axis. Each bar represents the probability of a sum, with the highest bar at T=7, showing \( \frac{1}{6} \). It will be symmetric since the probability distribution is symmetric around T=7.
05

Calculate and Interpret P(T ≥ 5)

To find \(P(T \geq 5)\), we add the probabilities from T=5 to T=12:\[P(T \geq 5) = P(T=5) + P(T=6) + P(T=7) + P(T=8) + P(T=9) + P(T=10) + P(T=11) + P(T=12) = \frac{4}{36} + \frac{5}{36} + \frac{6}{36} + \frac{5}{36} + \frac{4}{36} + \frac{3}{36} + \frac{2}{36} + \frac{1}{36} = \frac{26}{36} = \frac{13}{18}\]This result means there is a \( \frac{13}{18} \) probability that the sum of the dice will be at least 5.

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Key Concepts

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

Histogram
When dealing with probability distributions, a histogram is a useful visual tool. It allows you to easily see how probabilities are distributed.

A histogram for this dice-rolling problem has the sums of the dice (from 2 to 12) on the x-axis. The corresponding probabilities are plotted on the y-axis. Each bar in the histogram represents the probability of a specific sum occurring when rolling two six-sided dice.

Since rolling a 7 has the highest probability, this bar will be the tallest in the histogram. This visual shows us that sums around 7 occur more frequently compared to sums like 2 or 12. The distribution is symmetric around the peak at 7, reflecting the balanced likelihood of outcomes when rolling two dice.
Probability Calculation
Calculating probabilities for rolling a pair of dice involves understanding all possible outcomes. With two six-sided dice, there are 36 different possible outcomes (6 faces on the first die and 6 faces on the second die).

To calculate the probability of any specific sum, count how many outcomes result in that sum. For example, there’s only 1 way to roll a sum of 2 (both dice showing 1), so the probability is \(\frac{1}{36}\). For a sum of 7, there are 6 combinations (e.g., 1+6, 2+5), so its probability is \(\frac{6}{36} = \frac{1}{6}\).

This method of counting outcomes and dividing by the total possible outcomes is how we establish the probability distribution of the sums of dice rolls.
Interpretation of Probability
The probabilities calculated in this exercise tell us the likelihood of each possible sum when rolling two dice. These probabilities are useful in predicting outcomes over multiple rolls.

For instance, the calculation of \(P(T \geq 5)\) helps us understand the probability of rolling a sum of at least 5. This addition of individual probabilities from 5 to 12 results in \(\frac{13}{18}\).

This means there’s about a 72% chance of rolling a total sum of 5 or more with each throw of the dice. Such interpretations allow us to make informed decisions in games or mathematical simulations involving dice.

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