A random sample of 81 automobiles traveling on a section of an interstate showed an average speed of 60 mph. The distribution of speeds of all cars on this section of highway is normally distributed, with a standard deviation of 13.5 mph.

The value to use for the standard error of the mean is:

1.13.5
2.9
3.2.26
4.1.5

Let X the random variable that represent the variable of interest of a population, and for this case we know the distribution for X is given by:

Where and

And for this case we select a sample size of n= 81. Since the distribution for X is normal then we know that the distribution for the sample mean is given by:

And the standard error of the mean would be:

4.1.5

Step-by-step explanation:

Previous concepts

Normal distribution, is a "probability distribution that is symmetric about the mean, showing that data near the mean are more frequent in occurrence than data far from the mean".

The Z-score is "a numerical measurement used in statistics of a value's relationship to the mean (average) of a group of values, measured in terms of standard deviations from the mean".  

Solution to the problem

Let X the random variable that represent the variable of interest of a population, and for this case we know the distribution for X is given by:

Where and

And for this case we select a sample size of n= 81. Since the distribution for X is normal then we know that the distribution for the sample mean is given by:

And the standard error of the mean would be:

4.1.5

step-by-step explanation:

we are given

since, we have to isolate x on anyone side

so, we can move all x terms on one side

subtract both sides by 2x

add both sides by 9

divide both sides by 4

we get

6.5

step-by-step explanation:

you said it was in the question

~courtney