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.
Answer:

Step-by-step explanation:
Fixed monthly fee = $20
Amount charged per minute for long distance calls = $0.20
denotes total cost such that
minutes are used for long distance calls.
Amount charged for
minutes for long distance calls 
Therefore, an equation for the total cost, C, when N minutes are used for long distance calls is 
Answer:

Step-by-step explanation:
x² + 6x + 9 = 12 → x² + 6x - 3
Since this Quadratic Expression is unfactorable, apply the <em>Quadratic</em><em> </em><em>Formula</em>:


√48 = √[3 × 16] = 4√3
½[4√3] = 2√3
Then attach half of -6 to it as well [-3]:
-3 ± 2√3
I am joyous to assist you anytime.
The median for city A is 4 because the bracket is 2, 3.5, 4, 4, 5, 5.5. The middle numbers are 4 which add to eight and divide by two to make 4. The median for city B is 5. 25 because the bracket is 3.5, 4, 5, 5.5, 6, and 6. The middle numbers are 5 and 5.5 which add to 10.5 and divide by two to make 5.25.