Question

A section of highway is posted with a 55 mph speed limit. In fact, speeds of cars traveling on this section are normally distributed with mean 58 mph and standard deviation 5 mph. If the state patrol decides to ticket cars in the fastest 10 %, at what speed will they start ticketing

Answer 1

Answer:

$z=1.28$

And if we solve for a we got

$a=58 +1.28*5=64.4$

So the value of height that separates the bottom 90% of data from the top 10% is 64.4.  

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 velocity of a population, and for this case we know the distribution for X is given by:

$X \sim N(58,5)$  

Where $\mu=58$ and $\sigma=5$

For this part we want to find a value a, such that we satisfy this condition:

$P(X>a)=0.10$   (a)

$P(X$   (b)

Both conditions are equivalent on this case. We can use the z score again in order to find the value a.  

As we can see on the figure attached the z value that satisfy the condition with 0.90 of the area on the left and 0.1 of the area on the right it's z=1.28. On this case P(Z<1.28)=0.9 and P(z>1.28)=0.1

If we use condition (b) from previous we have this:

$P(X$  

$P(z$

But we know which value of z satisfy the previous equation so then we can do this:

$z=1.28$

And if we solve for a we got

$a=58 +1.28*5=64.4$

So the value of height that separates the bottom 90% of data from the top 10% is 64.4.