Question

The time I spend waiting for the bus on any given day has a distribution with mean 4 min- utes and variance off 0.5 minutes. What is the probability that I spend more than 2 hours and 10 minutes waiting for the bus in one month (30 days)? You may assume that waiting times on different days are independent of each other. HINT: Is there a sum of random variables somewhere in here?

Answer 1

Answer:

0.0049 = 0.49% probability that I spend more than 2 hours and 10 minutes waiting for the bus in one month.

Step-by-step explanation:

Normal Probability Distribution

Problems of normal distributions can be solved using the z-score formula.

In a set with mean $\mu$ and standard deviation $\sigma$, the z-score of a measure X is given by:

$Z = \frac{X - \mu}{\sigma}$

The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the p-value, we get the probability that the value of the measure is greater than X.

n instances of a normal variable:

For n instances of a normal variable, the mean is:

$M = n\mu$

$s = \sigma\sqrt{n}$

Mean of 4 minutes, standard deviation of 0.5 minutes:

This means that $\mu = 4, \sigma = \sqrt{0.5}$

30 days:

$M = 30(4) = 120$

$s = \sqrt{0.5}\sqrt{30} = \sqrt{0.5*30} = \sqrt{15}$

What is the probability that I spend more than 2 hours and 10 minutes waiting for the bus in one month (30 days)?

2 hours and 10 minutes is 2*60 + 10 = 130 minutes, so this probability is 1 subtracted by the p-value of Z when X = 130. So

$Z = \frac{X - \mu}{\sigma}$

In this context, due to the 30 instances of the normal variable:

$Z = \frac{X - M}{s}$

$Z = \frac{130 - 120}{\sqrt{15}}$

$Z = 2.58$

$Z = 2.58$ has a p-value of 0.9951.

1 - 0.9951 = 0.0049

0.0049 = 0.49% probability that I spend more than 2 hours and 10 minutes waiting for the bus in one month.