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\( X \) is a normally distributed random variable with mean 26 and standard deviation 9 . What is the probability that \( X \) is greater than \( 6 ? \) Write your answer as a decimal rounded to the nearest thousandth.

Ask by Howell Mccoy. in the United States
Jan 24,2025

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Answer

The probability that \( X \) is greater than 6 is **0.987**.

Solution

To determine the probability that \( X \) is greater than 6, where \( X \) is normally distributed with a mean (\( \mu \)) of 26 and a standard deviation (\( \sigma \)) of 9, follow these steps: 1. **Standardize the Variable:** Convert \( X \) to a standard normal variable \( Z \) using the formula: \[ Z = \frac{X - \mu}{\sigma} \] For \( X = 6 \): \[ Z = \frac{6 - 26}{9} = \frac{-20}{9} \approx -2.222 \] 2. **Find the Probability:** We need \( P(X > 6) \), which is equivalent to \( P(Z > -2.222) \). Using the symmetry of the normal distribution: \[ P(Z > -2.222) = P(Z < 2.222) \] 3. **Lookup the Standard Normal CDF:** Using standard normal distribution tables or a calculator: \[ \Phi(2.222) \approx 0.9865 \] 4. **Round to the Nearest Thousandth:** \[ 0.9865 \text{ rounded to the nearest thousandth is } 0.987 \] **Final Answer:** The probability that \( X \) is greater than 6 is **0.987**.

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To find the probability that \( X \) is greater than \( 6 \), we first need to standardize \( X \) using the z-score formula: \[ z = \frac{X - \mu}{\sigma} \] In this case, \( \mu = 26 \) (mean) and \( \sigma = 9 \) (standard deviation). For \( X = 6 \): \[ z = \frac{6 - 26}{9} = \frac{-20}{9} \approx -2.222 \] Next, we can use the standard normal distribution table or a calculator to find the probability that \( Z \) (the standard normal variable) is less than \( -2.222 \). This value is approximately \( 0.0132 \). Therefore, the probability that \( X \) is greater than \( 6 \) will be: \[ P(X > 6) = 1 - P(Z < -2.222) \approx 1 - 0.0132 = 0.9868 \] Rounding to the nearest thousandth, the probability that \( X \) is greater than \( 6 \) is: \[ \boxed{0.987} \]

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