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91Ó°ÊÓ

The probability that a 7-ounce skein of a discount worsted weight knitting yarn contains a knot is \(0.25 .\) Goneril buys ten skeins to crochet an afghan. a. Find the probability that (i) none of the ten skeins will contain a knot; (ii) at most one will. b. Find the expected number of skeins that contain knots. c. Find the most likely number of skeins that contain knots.

Short Answer

Expert verified
None have a knot: 0.0563, at most one: 0.244; expected: 2.5 skeins, most likely: 2 skeins.

Step by step solution

01

Understanding the Problem

We have a probability of 0.25 that a skein contains a knot. Goneril buys 10 skeins, and we are to determine certain probabilities and expectations using this information.
02

Identifying Distributions

Since we're dealing with a fixed number of trials (10 skeins) and discrete outcomes (knot or no knot), the situation fits a binomial distribution. The number of trials \(n = 10\) and the probability of success \(p = 0.25\).
03

Find Probability That None Will Contain a Knot

Using the binomial probability formula \(P(X = k) = \binom{n}{k} p^k (1-p)^{n-k}\), we find the probability that 0 skeins contain a knot: \(P(X = 0) = \binom{10}{0} (0.25)^0 (0.75)^{10}\). After calculating, \(P(X = 0) \approx 0.0563\).
04

Find Probability That At Most One Will Contain a Knot

"At most one" means finding \(P(X \leq 1)\). This is \(P(X = 0) + P(X = 1)\). Calculate \(P(X = 1) = \binom{10}{1} (0.25)^1 (0.75)^9\). Then add \(P(X = 0) \) and \(P(X = 1)\) to get \(P(X \leq 1) \approx 0.244 \).
05

Find Expected Number of Skeins with Knots

The expected value for a binomial distribution is \(E(X) = np\). Here \(E(X) = 10 \times 0.25 = 2.5\).
06

Find the Most Likely Number of Skeins with Knots

The most likely number is the mode of the distribution, approximated as \(\lfloor np + p \rfloor\). This results in \(\lfloor 2.5 + 0.25\rfloor = 2\).
07

Conclusion

We calculated that the probability of none containing knots is \(0.0563\), at most one is \(0.244\), the expected number is \(2.5\), and the most likely number is 2 skeins with knots.

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Key Concepts

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

Understanding Probability Calculation
Calculating probabilities, especially in binomial distribution scenarios, allows us to predict the likelihood of an event occurring a certain number of times. In our case, there's a probability of 0.25 that any one skein of yarn has a knot. Given Goneril has 10 skeins, we can express our situation with the binomial probability formula:\[ P(X = k) = \binom{n}{k} p^k (1-p)^{n-k} \]This formula helps determine how likely it is for exactly \( k \) skeins to have knots. Let's break it down:- \( \binom{n}{k} \) is the binomial coefficient, giving us the number of ways to pick \( k \) skeins that have knots out of the total \( n \) skeins.- \( p^k \) is the probability of \( k \) skeins having knots.- \( (1-p)^{n-k} \) is the probability that the remaining skeins do not have knots.Taking this into account for our examples: To find the probability of none containing knots (\( k = 0 \)), and at most one containing a knot (\( k \leq 1 \)), we calculated and found results as precise as \( P(X = 0) \approx 0.0563 \) and \( P(X \leq 1) \approx 0.244 \). Consider these probabilities as a guide to make decisions about the likelihood of knots showing up in your knitting project.
Grasping Expected Value
The expected value is a measure of the central tendency or "average" outcome in a random experiment. In the context of binomial distribution, it's a straightforward calculation: # Expected Value FormulaThe formula is:\[ E(X) = np \]Where:- \( n \) is the total number of trials (in this case, 10 skeins).- \( p \) is the probability of "success" (a skein having a knot, 0.25 here).For Goneril's yarn skeins:\[ E(X) = 10 \times 0.25 = 2.5 \]This means, on average, you can expect about 2.5 skeins out of 10 to have knots. Consider it as a weighted average reflecting real-life scenarios. You're unlikely to have fractions of skeins, but it's a mathematical expectation, guiding decisions and further probability predictions.
Exploring the Mode of Distribution
In probability and statistics, the mode is the value that appears most frequently in a data set. For the binomial distribution, the mode can tell you how many skeins with knots are most likely to occur.# Determining the ModeThe mode can be approximated using the formula:\[ \text{Mode} \approx \lfloor np + p \rfloor \]Here's how it works:- \( np \) gives an initial estimate (expected value).- Adding \( p \) accounts for the skewness in the distribution.- The floor function \( \lfloor \cdot \rfloor \) rounds down to the nearest whole number.In this exercise:\[ \lfloor 2.5 + 0.25 \rfloor = 2 \]Thus, the most likely number of skeins with knots is 2. It provides practical insight when analyzing results, highlighting the most frequent or probable outcome you would likely observe if this yarn scenario was repeated many times.

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