Question

1. the wait time at your local mcdonalds varies according to a normal distribution, with a mean of 7 minutes and standard deviation of 1.35 minutes. approximately what proportion customers wait times are over 9 minutes? (a) 0.033 (b) 0.069 (c) 0.189 (d) 0.191 (e) 0.221

Explanation:

Step1: Calculate the z - score

The z - score formula is $z=\frac{x-\mu}{\sigma}$, where $x = 9$, $\mu = 7$, and $\sigma=1.35$. So $z=\frac{9 - 7}{1.35}=\frac{2}{1.35}\approx1.48$.

Step2: Find the proportion

We want $P(X>9)$, which is equivalent to $P(Z > 1.48)$ for a standard normal distribution. Since the total area under the standard - normal curve is 1, and $P(Z>z)=1 - P(Z\leq z)$. Looking up $P(Z\leq1.48)$ in the standard normal table, we find $P(Z\leq1.48)\approx0.9306$. Then $P(Z > 1.48)=1 - 0.9306 = 0.0694\approx0.069$.

Answer:

B. 0.069