When there isn't an outlier, the mean is the best measure of central tendency, because it gives you a more arithmetic answer of the true center since all numbers are included in the equation rather than at max 2 numbers in the middle of the list.
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.
Its choice d because 1^2 - 5(1) + 6
= 1 - 5 + 6
= 2
1 = 20 (24-4(1)=20)
2 = 16 (24-4(2)=16)
5 = 4 (24-4(5)=4)
6 = 0 (24-4(6)=0)