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\( X \) is a normally distributed random variable with mean 58 and standard deviation 2 . What is the probability that \( X \) is greater than 60 ? 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 Byrd Bowen. in the United States
Jan 24,2025

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The probability that \( X \) is greater than 60 is **0.160**.

Solution

To determine the probability that \( X \) is greater than 60, where \( X \) is normally distributed with a mean (\( \mu \)) of 58 and a standard deviation (\( \sigma \)) of 2, we can use the **68-95-99.7 rule** (also known as the empirical rule). 1. **Identify the Z-score:** \[ Z = \frac{X - \mu}{\sigma} = \frac{60 - 58}{2} = 1 \] So, 60 is 1 standard deviation above the mean. 2. **Apply the 68-95-99.7 Rule:** - **68%** of the data falls within **±1** standard deviation from the mean. - Therefore, the probability that \( X \) is **less than or equal to 60** is roughly **68% / 2 = 34%** above the mean. - The total area under the curve is 100%, so the probability that \( X \) is **greater than 60** is: \[ 50\% - 34\% = 16\% \] 3. **Convert to Decimal:** \[ 16\% = 0.16 \] **Final Answer:** The probability that \( X \) is greater than 60 is **0.160**.

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To find the probability that \( X \) is greater than 60 using the \( 0.68-0.95-0.997 \) rule, we first calculate how many standard deviations away 60 is from the mean. The mean is 58, and the standard deviation is 2, so: \[ Z = \frac{X - \mu}{\sigma} = \frac{60 - 58}{2} = 1. \] According to the rule, about 68% of the data lies within one standard deviation of the mean (between 56 and 60). Thus, the probability of \( X \) being less than 60 is 0.5 (50% of the area under the curve to the left of the mean) plus half of the 68%, which gives us: \[ P(X < 60) = 0.5 + 0.34 = 0.84. \] To find the probability that \( X \) is greater than 60, we subtract this from 1: \[ P(X > 60) = 1 - P(X < 60) = 1 - 0.84 = 0.16. \] So, the probability that \( X \) is greater than 60 is \( 0.160 \).

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