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It is known that \(60 \%\) of mice inoculated with a serum are protected from a certain disease. If 5 mice are inoculated, find the probability that (a) none contracts the discase: (b) fewer than 2 contract the disease; (c) more than 3 contract the disease.

Short Answer

Expert verified
(a) The probability that no mouse contracts the disease is 0.01024. (b) The probability that fewer than 2 mice contract the disease is 0.11264. (c) The probability that more than 3 mice contract the disease is 0.33696.

Step by step solution

01

Understanding the Variables

We define a 'success' as a mouse being protected from the disease. The number of trials, \(n\), is 5, which is the number of mice. The probability of success, \(p\), is \(0.60\). The probability of failure, \(q\), is therefore \(1.0-0.60=0.40\).
02

Calculate the Probability for (a)

We want to calculate the probability that no mouse contracts the disease, which means all are protected. This is a simple case of the binomial distribution where \(x=0\). We use the formula \(Pr(X=k) = C(n, k) * p^k * q^{n-k}\). So \(Pr(X=0) = C(5, 0) * (0.60)^0 * (0.40)^5 = 0.01024\).
03

Calculate the Probability for (b)

Now we want to calculate the probability that fewer than 2 mice contract the disease, which means 0 or 1 mouse contracts the disease. This requires summing the binomial probabilities for 0 and 1: \(Pr(X<2) = Pr(X=0) + Pr(X=1) = C(5, 0)*(0.60)^0 * (0.40)^5 + C(5, 1)*(0.60)^1 * (0.40)^4 = 0.01024 + 0.10240 = 0.11264\).
04

Calculate the Probability for (c)

Finally, we want to calculate the probability that more than 3 mice contract the disease, meaning 4 or 5 mice contract the disease. This also requires summing the binomial probabilities for 4 and 5: \(Pr(X>3) = Pr(X=4) + Pr(X=5) = C(5, 4)*(0.60)^4 * (0.40)^1 + C(5, 5)*(0.60)^5 * (0.40)^0 = 0.25920 + 0.07776 = 0.33696\).

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Key Concepts

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

Probability Theory
Probability theory is the branch of mathematics that deals with uncertainty and randomness. It helps us calculate the likelihood of different outcomes in various situations. In this exercise, we apply probability theory to understand events in a binomial distribution. This involves determining the probability of a certain number of 'successes' (or events of interest) occurring across a fixed number of trials. For our exercise, a 'success' is when a mouse is protected from a disease after being inoculated, with a probability of success given as 0.60.By using the binomial distribution formula, \(Pr(X=k) = C(n, k) * p^k * q^{n-k}\), where \(n\) is the number of trials and \(p\) and \(q\) are the probabilities of success and failure, respectively, we calculate the probability of each possible outcome related to the mice being protected from the disease. This formula helps determine specific scenarios such as all mice being protected or specific numbers of them contracting the disease.
Statistical Analysis
Statistical analysis allows us to interpret data and make inferences about populations or experiments. In this problem, we have a small sample of five mice, and we're using statistical techniques to predict the number that may or may not contract a disease. By evaluating different probabilities such as none, fewer than two, or more than three mice contracting the disease, we're essentially building a statistical understanding of the effectiveness of the serum administered. This process involves not only calculating point probabilities (like "no mice contract the disease"), but also cumulative probabilities (such as "fewer than 2 mice contract the disease").
  • Point Probability: Calculating the likelihood of a specific number of events (for example, exactly zero mice contracting the disease).
  • Cumulative Probability: Finding probabilities for a range of outcomes, often needing the sum of multiple probabilities (like at most one mouse contracting the disease).
These calculations provide insights into the overall performance and reliability of the inoculation strategy being tested.
Combinatorics
Combinatorics forms the backbone of calculating probabilities in the binomial distribution by determining how combinations (the number of ways of picking items) affect probability. In a binomial distribution scenario like ours, combinatorics is used to determine the different ways 'successes' can occur among our trials of mice.The key combinatorial component in our solution is the binomial coefficient, expressed as \(C(n, k)\), which calculates the number of combinations possible when choosing \(k\) successes from \(n\) trials. This is given by the formula:\[C(n, k) = \frac{n!}{k!(n-k)!}\]In our exercise, if we want to calculate the probability of exactly zero mice contracting the disease, \(C(5, 0)\) determines the number of ways zero events of contraction can happen.Thus, combinatorics enables us to break down the problem into manageable parts, calculate probabilities for specific numbers of "successes," and adjust for different scenarios by summing these probabilities as necessary. This enables a thorough analysis of the probability landscape outlined by this binomial distribution.

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