The z-score associated with 14.3 is 0.84. 0.2995 of the population is between 12.2 and 14.3. 0.1894 of the population is less than 10.0.
The formula for a z-score is
z=(X-μ)/σ
With our data, we have: z=(14.3-12.2)/2.5=0.84
The z-score associated with the mean is 0.5. To find the proportion of the population between the mean and 14.3, subtract 0.7995 (the proportion of population below the z-score of 0.84, using http://www.z-table.com) and 0.5:
0.7995 - 0.5 = 0.2995.
The z-score for 10.0 is
(10.0-12.2)/2.5 = -0.88. The proportion of the population less than this is 0.1894.
