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Chapter 3: Problem 80

A coin that comes up heads with probability \(p\) is flipped \(n\) consecutive times. What is the probability that starting with the first flip there are always more heads than tails that have appeared?

Short Answer

Expert verified
The probability of having more heads than tails at any point in the sequence of n flips (even n only) is given by: \[P(\text{more heads than tails}) = \frac{C_{n/2} \cdot p^{n/2 + 1} \cdot (1-p)^{n/2 - 1}}{2^n}\] Where \(C_{n/2}\) is the Catalan number for \(\frac{n}{2}\) and p is the probability of getting a head. For any odd n, the probability will be 0.

Step by step solution

01

Determine all possible outcomes

The first step is to determine the total number of possible outcomes for flipping a coin n times. Since there are 2 possible outcomes (heads and tails) for each flip, there are a total of 2^n possible outcomes.
02

Valid sequences

Now, we need to determine the number of valid sequences where we have more heads than tails at any given point. A valid sequence can only start with a head. For a sequence of length n, we have the following number of valid sequences: - If n is odd, there cannot be a valid outcome since there will always be an even number of tails. - If n is even, we can calculate the valid sequences using the Catalan numbers which are given by the formula \(C_{k} = \frac{1}{k+1} \binom{2k}{k}\), where k represents the position in the sequence of the Catalan numbers. In our case, k = n/2.
03

Calculate probability

To find the probability, we can divide the number of valid sequences by the total number of outcomes. Since we're given that the probability of getting a head on any given flip is p, the probability of getting a tail is (1-p). For the number of valid sequences, we use the Catalan number with k = n/2, as derived in the previous step. So, the probability of having more heads than tails at any point in the sequence is: \[P(\text{more heads than tails}) = \frac{C_{n/2} \cdot p^{n/2 + 1} \cdot (1-p)^{n/2 - 1}}{2^n}\] Keep in mind that the formula is valid only for even n, for any odd n the probability will be 0.

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Most popular questions from this chapter

A set of \(n\) dice is thrown. All those that land on six are put aside, and the others are again thrown. This is repeated until all the dice have landed on six. Let \(N\) denote the number of throws needed. (For instance, suppose that \(n=3\) and that on the initial throw exactly two of the dice land on six. Then the other die will be thrown, and if it lands on six, then \(N=2 .\) ) Let \(m_{n}=E[N]\). (a) Derive a recursive formula for \(m_{n}\) and use it to calculate \(m_{i}, i=2,3,4\) and to show that \(m_{5} \approx 13.024\). (b) Let \(X_{i}\) denote the number of dice rolled on the \(i\) th throw. Find \(E\left[\sum_{i=1}^{N} X_{i}\right]\).

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