I'm assuming that you mean
because if you meant
then u would simplify and you couldn't make it the subject.
Under this assumption, we start with
We multiply both sides by
We expand the left hand side:
We move all terms involving u to the left and all terms not involving u to the right:
We factor u on the left hand side:
We divide both sides by
Answer:
And we can find this probability using the normal standard table or excel and we got:
The figure shows the calculation for this case.
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 hardness of a population, and for this case we know the distribution for X is given by:
Where and
We are interested on this probability
And the best way to solve this problem is using the normal standard distribution and the z score given by:
If we apply this formula to our probability we got this:
And we can find this probability using the normal standard table or excel and we got:
The figure shows the calculation for this case.
