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Suppose that a biased coin that lands on heads with probability \(p\) is flipped 10 times. Given that a total of 6 heads results, find the conditional probability that the first 3 outcomes are (a) \(h, t, t\) (meaning that the first flip results in heads, the second in tails, and the third in tails); (b) \(t, h, t\)

Short Answer

Expert verified
(a) The conditional probability for h, t, t in the first three flips is: P(h, t, t | 6 heads) = \(\frac{p * (1-p)^2 * \binom{7}{5} * p^5 * (1-p)^2}{\binom{10}{6} * p^6 * (1-p)^4}\) (b) The conditional probability for t, h, t in the first three flips is: P(t, h, t | 6 heads) = \(\frac{(1-p) * p * (1-p) * \binom{7}{5} * p^5 * (1-p)^2}{\binom{10}{6} * p^6 * (1-p)^4}\)

Step by step solution

01

Compute binomial probability for 6 heads in 10 flips

To compute the probability of obtaining exactly 6 heads in 10 flips, we need to use the binomial probability formula: P(X=k) = \(\binom{n}{k}\) * \(p^k\) * \((1-p)^{n-k}\) In this case, X is the number of heads, n is the number of flips (10), k is the desired number of heads (6), and p is the probability of obtaining a head. Plugging in our values, we get: P(X=6) = \(\binom{10}{6}\) * \(p^6\) * \((1-p)^{4}\) Step 2: Determine the probability of the given scenarios (a) Probability for h, t, t in the first three flips
02

Compute binomial probability for remaining flips and scenario (a) probability

Since the first flip has one head, we need to find the probability of obtaining 5 heads in the remaining 7 flips. We can again use the binomial probability formula: P(Y=5) = \(\binom{7}{5}\) * \(p^5\) * \((1-p)^{2}\) Now, we need to find the probability of the specific sequence h, t, t, followed by 5 heads in the remaining 7 flips: P(h, t, t | 6 heads) = \(P(h) * P(t) * P(t) * P(Y=5)\) P(h, t, t | 6 heads) = \(p * (1-p) * (1-p) * \binom{7}{5} * p^5 * (1-p)^2\) (b) Probability for t, h, t in the first three flips
03

Compute binomial probability for remaining flips and scenario (b) probability

In this scenario, we have one head in the first three flips, so we need to find the probability of obtaining 5 heads in the remaining 7 flips, which is the same as in (a): P(Y=5) = \(\binom{7}{5}\) * \(p^5\) * \((1-p)^{2}\) Now, we need to find the probability of the specific sequence t, h, t, followed by 5 heads in the remaining 7 flips: P(t, h, t | 6 heads) = \(P(t) * P(h) * P(t) * P(Y=5)\) P(t, h, t | 6 heads) = \((1-p) * p * (1-p) * \binom{7}{5} * p^5 * (1-p)^2\) Step 3: Compute the conditional probabilities (a) Conditional probability for h, t, t in the first three flips
04

Compute conditional probability for scenario (a)

To find the conditional probability for scenario (a), divide the probability of scenario (a) by the probability of 6 heads in 10 flips: P(h, t, t | 6 heads) = \(\frac{p * (1-p) * (1-p) * \binom{7}{5} * p^5 * (1-p)^2}{\binom{10}{6} * p^6 * (1-p)^4}\) (b) Conditional probability for t, h, t in the first three flips
05

Compute conditional probability for scenario (b)

Similarly, to find the conditional probability for scenario (b), divide the probability of scenario (b) by the probability of 6 heads in 10 flips: P(t, h, t | 6 heads) = \(\frac{(1-p) * p * (1-p) * \binom{7}{5} * p^5 * (1-p)^2}{\binom{10}{6} * p^6 * (1-p)^4}\) Now you have found the conditional probabilities for each scenario.

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Key Concepts

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

Binomial Probability
Binomial probability is a type of probability calculation that is typically used when we have an experiment with two possible outcomes for each trial, known as 'success' and 'failure'. To compute binomial probabilities, the trials must be independent, meaning the outcome of one trial does not affect the other. Additionally, the probability of success must remain the same for each trial.

Let's consider the provided exercise where a biased coin is flipped several times, and we are interested in the probability of getting a certain number of heads. This is a scenario well-suited for binomial probability. The general formula to compute the probability of getting exactly k successes (heads, in our context) in n independent trials is given by:
P(X=k) = \(\binom{n}{k}\) * \(p^k\) * \((1-p)^{n-k}\)

This formula involves the use of combinatorics, specifically the binomial coefficient \(\binom{n}{k}\), which calculates the number of ways to choose k successes in n trials. Here, \(p\) is the probability of success on a single trial, and \(1-p\) is the probability of failure. Knowing how to apply this formula is essential for solving problems that follow a binomial distribution.

When improving the understanding of this concept, it is important to visualize the process and outcomes of each trial. To increase comprehension, you can use diagrams or 'trees' that represent the possible outcomes of flipping the coin, which aids in understanding the binomial distribution and how it applies to multiple trials in a probabilistic experiment.
Probability Theory
Probability theory is the branch of mathematics that deals with the analysis of random events. The main objective is to determine the likelihood of occurrence for different outcomes of an event. It is a fundamental concept used in various fields, such as finance, insurance, and statistics.

In our problem, we're using probability theory to understand the likelihood of certain sequences occurring given that we know 6 heads were obtained in 10 flips of a biased coin. The calculations leverage rules of probability like independence of events and the multiplication rule, which states that the probability of two independent events occurring together is the product of their individual probabilities.

For example, the probability of the sequence 'h, t, t' can be expressed as the product of the probabilities of each individual event occurring, assuming they are independent. That is, the probability of getting a head first, then a tail, and another tail. In doing so, we are applying core principles of probability theory to work out the likelihood of this particular sequence.

The concept is further explored when we consider conditional probability, which is the probability of an event occurring given that another event has already occurred. For useful and improved understanding of this theoretical framework, students should frequently practice with real-world scenarios, as this helps bridge the gap between abstract formulas and tangible applications.
Combinatorics
Combinatorics is the area of mathematics concerned with counting, combination, and permutation of sets of elements, and is a foundational piece of many probability problems. In the context of our coin-flipping exercise, we use combinatorics to determine the number of ways we can achieve a certain number of heads (successes) in a series of coin flips.

To understand the combinatoric aspect, consider the binomial coefficient \(\binom{n}{k}\) we encountered in the binomial probability formula. This coefficient represents the number of ways to choose k successes out of n total trials, which is a combinatorial calculation. It is also known as 'n choose k'.

Combinatorics scales in complexity with the number of elements, so when explaining this to students, starting with simple, tangible examples can be beneficial. Employing visual aids, such as Pascal's Triangle or listing out combinations, can make the concept easier to grasp. Additionally, working through the process of calculating a coefficient step-by-step solidifies understanding of the combinatorial principles in play. By practicing these methods, students gain insight into the mechanics behind the probability of compound events and develop a deeper understanding of how outcomes are determined in probabilistic scenarios.

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