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\( X \) is a normally distributed random variable with mean 35 and standard deviation 11 . What is the probability that \( X \) is greater than 55 ? Write your answer as a decimal rounded to the nearest thousandth.

Ask by Young Moreno. in the United States
Jan 24,2025

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Answer

The probability that \( X \) is greater than 55 is approximately 0.034.

Solution

To find the probability that \( X \) is greater than 55 for a normally distributed random variable with mean \( \mu = 35 \) and standard deviation \( \sigma = 11 \), we can use the standard normal distribution (Z-score). ### Step 1: Calculate the Z-score The Z-score is calculated using the formula: \[ Z = \frac{X - \mu}{\sigma} \] Substituting the values: \[ Z = \frac{55 - 35}{11} = \frac{20}{11} \approx 1.8182 \] ### Step 2: Find the probability Next, we need to find the probability that \( Z \) is greater than 1.8182. This can be found using the standard normal distribution table or a calculator. \[ P(Z > 1.8182) = 1 - P(Z \leq 1.8182) \] ### Step 3: Look up the cumulative probability Using a standard normal distribution table or calculator, we find: \[ P(Z \leq 1.8182) \approx 0.9656 \] ### Step 4: Calculate the final probability Now we can calculate: \[ P(Z > 1.8182) = 1 - 0.9656 = 0.0344 \] ### Final Answer Thus, the probability that \( X \) is greater than 55 is approximately: \[ \boxed{0.034} \] This value is rounded to the nearest thousandth.

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To find the probability that \( X \) is greater than 55, we can use the Z-score formula: \[ Z = \frac{X - \mu}{\sigma} \] where \( \mu = 35 \) and \( \sigma = 11 \). Plugging in the values: \[ Z = \frac{55 - 35}{11} = \frac{20}{11} \approx 1.818 \] Next, we need to look up the Z-score in a standard normal distribution table or use a calculator to find the probability. The Z-score of 1.818 corresponds to a cumulative probability of approximately 0.964. This means that the probability \( P(X < 55) \approx 0.964 \). To find \( P(X > 55) \): \[ P(X > 55) = 1 - P(X < 55) = 1 - 0.964 \approx 0.036 \] So, the probability that \( X \) is greater than 55, rounded to the nearest thousandth, is \( \boxed{0.036} \).

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