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For a binomial probability distribution, \(n=120\) and \(p=.60\). Let \(x\) be the number of successes in 120 trials. a. Find the mean and standard deviation of this binomial distribution. b. Find \(P(x \leq 69)\) using the normal approximation. c. Find \(P(67 \leq x \leq 73)\) using the normal approximation.

Short Answer

Expert verified
a. Mean = 72, Standard Deviation ≈ 6.93, b. P(x ≤ 69) ≈ 0.3336 using the normal approximation, c. P(67 ≤ x ≤ 73) ≈ 0.3199 using the normal approximation.

Step by step solution

01

Calculate Mean and Standard Deviation

The mean and standard deviation of a binomial distribution can be calculated using the formula:Mean (\( \mu \)) = np, Standard Deviation (\( \sigma \)) = \sqrt{np(1-p)}Here, n = 120 and p = 0.60. Therefore, the mean (\( \mu \)) = np =120*0.60 = 72 and the standard deviation (\( \sigma \)) = \sqrt{np(1-p)}= \sqrt{120 * 0.60 * 0.40} ≈ 6.93.
02

Calculate P(x ≤ 69) using Normal Approximation

For P(x ≤ 69) using the Normal Approximation, first convert x to a z-score using the formula: z = (x - μ) / σ. Here, x = 69, so z = (69 - 72) / 6.93 ≈ -0.43. Using a standard z-table, look up the area to the left of z = -0.43 to find the probability. Assume a z-table gives a probability of 0.3336 which corresponds to P(x ≤ 69).
03

Calculate P(67 ≤ x ≤ 73) using Normal Approximation

For P(67 ≤ x ≤ 73), calculate two z-scores: z1 for x = 67 and z2 for x = 73. z1 = (67 - 72) / 6.93= -0.72 and z2 = (73 - 72) / 6.93 = 0.14. Using a z-table, find the probability for z1 and z2. Let's say the probabilities from the z-Table are 0.2358 and 0.5557 respectively. The probability P(67 ≤ x ≤ 73) = probability of z2 - probability of z1 = 0.5557 - 0.2358 = 0.3199.

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