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\( X \) is a normally distributed random variable with mean 37 and standard deviation 4 . What is the probability that \( X \) is less than 29 ? Use the \( 0.68-0.95-0.997 \) rule and write your answer as a decimal. Round to the nearest thousandth if necessary.

Ask by Ruiz Colon. in the United States
Jan 24,2025

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Answer

The probability that \( X \) is less than 29 is **0.025**.

Solution

To determine the probability that \( X \) is less than 29, where \( X \) is normally distributed with a mean (\( \mu \)) of 37 and a standard deviation (\( \sigma \)) of 4, we can use the \( 68-95-99.7 \) (empirical) rule. ### Step-by-Step Calculation: 1. **Find the Z-score:** \[ Z = \frac{X - \mu}{\sigma} = \frac{29 - 37}{4} = \frac{-8}{4} = -2 \] 2. **Interpret the Z-score using the \( 68-95-99.7 \) rule:** - The rule states: - Approximately **68%** of data falls within **±1** standard deviation. - Approximately **95%** within **±2** standard deviations. - Approximately **99.7%** within **±3** standard deviations. - A Z-score of **-2** corresponds to the lower 2 standard deviations from the mean. 3. **Determine the probability:** - Since \( Z = -2 \) is two standard deviations below the mean, the probability of \( X \) being less than 29 is roughly **2.5%**. 4. **Expressing the probability as a decimal:** \[ 0.025 \] ### Final Answer: The probability that \( X \) is less than 29 is **0.025**.

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To find the probability that \( X \) is less than 29, we can use the properties of the normal distribution. First, we need to determine how many standard deviations away 29 is from the mean. The mean \( \mu = 37 \) and the standard deviation \( \sigma = 4 \). We calculate the z-score for \( X = 29 \): \[ z = \frac{X - \mu}{\sigma} = \frac{29 - 37}{4} = \frac{-8}{4} = -2 \] A z-score of -2 means that 29 is 2 standard deviations below the mean. According to the 68-95-99.7 rule, about 95% of the data lies within 2 standard deviations from the mean, which means: - 2.5% lies below \( \mu - 2\sigma \) (which corresponds to a z-score of -2). - 2.5% lies above \( \mu + 2\sigma \). Thus, the probability that \( X \) is less than 29 is approximately 0.025. So, the answer is \( 0.025 \).

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